20-Inch vs 18-Inch Wheels and Tires

20-inch vs. 18-inch Cooper Discoverer AT3 XLT heavy-duty application tire evaluation.

Tires As Suspension

In the high-performance car (and even sport-truck) world it’s obvious that tires are a critical part of the suspension setup and the overall dynamics and handling of a vehicle. In the heavy-duty pickup world tires should be regarded with similar importance regarding how they handle loads, although it seems this is frequently ignored or overlooked until a problem arises. Such laissez-faire attitude seems more prevalent with those who infrequently work their trucks.

Unless one is never going to load their rig, generally aftermarket tire-and-wheel combinations should not have less capacity than the originals. However, just because a tire can support a particular load does not mean it is providing optimal performance. There are many potential differences, which include the tire size, load index, maximum psi, construction, tread pattern and compound, etc. Here is a real-world example of how dramatically tires can affect handling?

Surefooted Mule To Wallowing Pig

While driving to the 2019 Overland Expo West show last May, my ‘Mule started handling poorly. What I discovered, arriving in Flagstaff, Arizona, was that I had lost a bolt clamping the anti-roll bar to the rear axle. This rendered the bar useless and explained the sketchy handling I was experiencing.

On a coil-sprung vehicle, particularly one with a maximum and tall load, body-roll bars can be critical for safe handling, particularly during emergency maneuvers. To mitigate the then unexplained sloppy handling, twice I stopped to increase tire pressures. Adding 20 psi and over-inflating the 65-psi-maximum tires made a slight but still noticeable stability improvement. It firmed-up the relatively tall and flexible 35” tire sidewalls on 17” wheels, partially compensating for the looseness of the disconnected rear body-roll bar.

Missing bolt and bent Hellwig Big Wig rear stabilizer bracket explained the loose handling.

Thankfully my friends at Factor 55 had a block of titanium on which we pounded the deformed bushing bracket back into a usable shape. Another friend had a spare fastener. Seems losing anti-sway bar bolts on late-model Ram 2500s is not that uncommon. My friend was carrying a spare bolt because he had previously lost one himself; we both have the same Hellwig Big Wig rear bar. Another acquaintance at the event had also found a bolt missing on his factory rear bar while in Death Valley. So owners of newer Rams might want to check their sway-bar hardware, add some Loctite, and torque ‘em.

Air Supports Weight

It’s important to remember that it’s the total amount of air in a tire that supports weight; this includes the physical volume combined with the psi. This is one reason why some of the super-short sidewall tires are not appropriate for heavy-duty pickups; the ratings can be ridiculously low, partially because there is not enough physical space for adequate compressed air.

Unreasonably short sidewalls (think low-rider) will provide a firm, sometimes even jarring ride over small imperfections in the roadway, and in extreme cases potholes might bend a wheel because the thin sidewall does not provide adequate impact absorption. With that visual in mind, it can be helpful to think of tires as the biggest shock absorbers (dampers) on trucks. They can work in our favor or against us. Softer, tall-sidewalls can absorb impacts and improve ride quality for one application, or reduce control and handling on another. Adjusting the pressure or changing the size can increase or decrease overall performance.

Large 37×12.50R17LT Goodyear Wrangler MT/R is only a 50 psi tire, but the tire volume makes the SRW rating 3,525-lb each. I generally prefer the handling attributes of higher psi tires on heavy rigs. This outfit is a good an example of excessive tail-swing and rear overhang, both all too common on many modern flatbed camper builds.

My dislike for huge wheels with silly shortsidewalls on four-wheel-drive pickups emanates from decades of actually using trucks for off-pavement travel instead owning them as a fashion statement. What constitutes a silly short sidewall on a four-wheel-drive depends on the diameters of both the wheel and tire, but historically an aspect ratio below 70-percent might qualify. However, the aspect ratio alone does not tell the whole story, as it is a percentage of the width. A bigger wheel and tire combination with a lower aspect ratio might still have enough sidewall to work well off-pavement, while providing superior control on-road, particularly with a massive load.

Function Over Form

There’s little doubt that the humongous big-wheel craze on pickups (20” and taller) is mostly the result of the street-truck culture, not farmers and ranchers, heavy haulers, or commercial industries. Brakes are much larger on modern vehicles, but it doesn’t take 20” wheels to clear them. My recent interest in bigger wheels with shorter sidewall rubber is for optimal chassis stability and performance with my Hallmark camper; I prefer the traditional look of more tire and less wheel.

Many late-model diesel pickups are delivered with thirty-threes mounted on 18” wheels, and have a 7.5” sidewall. (Overall diameter minus the wheel height divided by two equals sidewall height.) Increasing tire diameter to 35” on an 18” wheel produces an 8.5” sidewall, an almost 14% increase. The same 35” rubber on a 17” wheel produces a 9” sidewall, which is a substantial 20-percent increase over the stock LT275/70R18 size. Using a 20” wheel for a thirty-five brings the sidewall height back down to 7.5”; same as the factory thirty-three on an 18” rim.

People don’t complain about the stock 275/70R18E having a short sidewall. SRW capacity is a respectable 3,640-lb at 80 psi.

If you think a 35” tire is huge, think again. Most new heavy-duty Rams will fit a narrow 35” tire (LT285/75R18) on stock wheels with little or no rubbing, and only minor rubbing on the radius arms at full-steering-lock if a wider, 35×12.50R18 is squeezed onto the factory rims. Running only 35s on my fourth-generation Ram/Cummins 2500s puts me in the smaller tire club; 37s on aftermarket wheels are the popular cool choice for many.

Cooper’s AT3 XLT in 285/75R18E (34.84”) with a 129 load index can handle 4,080-lb each at 80 psi.

Are 19.5” Wheels and Medium-Duty Tires The Answer?

Some of the heavy-hauling crowd, particularly those with cab-over campers, found a solution decades ago with 19.5” medium-duty commercial tires when there were fewer tire size options. Rickson Wheel Manufacturing was founded to service this niche. The enthusiast owner started making 19.5” SRW 8-lug wheels to fit heavy-duty pickups because he was burning through sets of 16” rubber on his camper-equipped second generation Dodge Cummins turbo-diesel. This was before the excellent availability of 18” and 20” light-truck tires and wheels, which can offer up to 35% more capacity per tire than the sixteens of the 1990s.

In addition to superior load support and firmer handling provided by the much stiffer 19.5” components, there are theoretical benefits of substantially more longevity. I have yet to find a well-documented, back-to-back comparison proving similar tread design medium-duty rubber  lasting much longer than a commercial-grade modern LT tire. I don’t doubt that it is possible, but show me. Medium-duty tires typically cost more, so more miles may be needed to make the modification pencil-out for those wanting to save money. Tread choices are also fewer.

In addition to limited availability of 19.5 inch wheels for pickups, and the possibly too firm load range F, G, or H construction, the biggest negative I see with medium-duty rubber are the lower speed ratings. Up to 245/70R19.5, which is only a 33” tall tire, the speed rating is a reasonable 87 mph. Moving up to a 265/70/19.5 or larger size typically reduces the speed rating to 75 mph.

Are 19.5” tires and wheels the answer for heavy pickups? They are for some heavy haulers.

There’s one area where the folks who favor 19.5″ tires and wheels and my recent interest in modern heavy-duty light-truck 20″ tires align perfectly; it’s about how they work not their appearance.

If one is going to load their SRW chassis with more than 8,160-pounds on the rear axle, the capacity offered by load-index 129 light-truck tires, super-stout and less flexible medium-duty rubber might be necessary and/or beneficial. I have read several comments online, and spoken to others who say 19.5” tires are too stiff over rough roads, both paved and not, even on heavy trucks. On unloaded rigs, the ride is sometimes described as downright jarring. Some are willing to tolerate an extra-firm ride, but is worth remembering that unnecessary roughness will be transmitted to all chassis components, potentially reducing their longevity. Surely some folks love their 19.5” tires and wheels on SRW pickups, but they appear to be a small minority.

If someone wants to lend me a set I’d be happy to bolt them on and give them a spin. My extensive heavy-duty pickup and LT tire knowledge has repeatedly convinced me that I don’t need to spend my time and money on medium-duty rubber for my pickups. Skipping the 19.5” wheels and going straight to modern, readily available twenties made more sense.

Cooper Discoverer AT3 XLT 18” vs. 20”

In typical RoadTraveler James fashion, I did back-to-back evaluation drives to observe differences between almost identical rubber wrapped around different diameter wheels. To limit controllable variables, the same manufacturer and tread design was chosen. Without this approach, carcass construction differences and other factors would unfairly skew impressions. While construction matters (continue reading), sometimes much, the goal was to test what dynamics are affected by simply changing the wheel diameter. With that established, one can then seek tire construction and/or other suspension solutions as needed for their specific application.

18” wheel on the left, 20” wheel on the right, all other specs are very similar.

The good folks at Cooper Tires, one of the few American companies still making tires in the United States, supplied the rubber to facilitate the evaluations. The redesigned Cooper Discoverer AT3 Family of Tires was introduced at the 2018 SEMA Show. It includes three distinct models targeted at specific segments of the all-terrain market. Cooper describes the attributes of these AT3s as:

Discoverer AT34S™ Severe snow rated with improved wet weather performance, snow performance, fuel economy, and with significantly improved tread wear.

Discoverer AT3LTand  Discoverer AT3XLT™ Designed to be powerful, shred-resistant all-terrain tires. With the Discoverer A/T3™ well-known for its long-lasting tread wear and dirt road performance, the new Discoverer AT3LT™ and  Discoverer AT3XLT™ tires feature Durable-Tread Technology™, enhancing durability, and enabling a best-in-class mileage warranty.

Longevity and durability has much to do with the application and duty-cycle, but Cooper has certainly upgraded and expanded the all-terrain AT3 line to cover all consumers who want this type of five-rib tire. Heavy-duty pickup owners should look at the AT3 XLT version, as it comes in the largest sizes with the highest load ratings.

Similar But Different

Because I prefer taller and narrower sizes for most conditions, I chose the 34.84-inch-tall LT285/75R18, and the 34.57-inch-tall LT285/65R20. There is very little difference in height or width between these two sizes, for all practical purposes they are the same, with the exception of the wheel diameter, which gives the twenty a much shorter sidewall. The 285/75R18 has a 8.42” sidewall, and the 285/65R20 has a 7.28” sidewall. Both sets of tires were mounted on factory Fourth Generation Ram aluminum wheels.

Both tires have a 285mm section width, but look at the tread width difference, which is noted in Cooper’s specifications.
Cooper Discoverer AT3 XLT LT285/65R20 LT285/75R18
Load Index (SRW) 127 = 3,860-lb 129 = 4,080-lb
Height 34.57” 34.84”
Section Width 11.5” 11.5”
Tread Width 9.69” 8.9”
Weight 62-lb 60-lb
Tread Depth 16/32” 17/32”

Cooper AT3 XLT General Observation

Both sizes track straight and true, as one would expect with a five-rib all-terrain. While driving through dozens of miles in heavy rain, the AT3XLT performed fantastically, evacuating water well, a forte of a ribbed tread pattern. Siping is generous to help with all slippery conditions. Though obviously not a mudder, some light, shallow mud was handled impressively well during a weeklong autumn camping trip in remote northeast Nevada.

Unless one wants a dedicated highway offering like the Cooper HT3, the AT3 XLT is a quiet, efficient design that should yield good fuel economy. The XLT has attractive, aggressive sidewall shoulder tread that looks good and should prevent both off-pavement and curb scrubbing damage.

Sitting at 8,900’ after two days off-pavement and some recent light mud, the Cooper AT3 XLT performed well.

Test Drives

Several back-to-back drives were performed over six months. The first with more off-pavement miles was a 45-mile loop that included 30 miles of freeway, five miles of rural highway, and 10 miles of rocky dirt on the historic Henness Pass Road trans-Sierra route. When the road got rough, I continued for a few miles at full psi, before dramatically lowering the pressure at a specific point to improve the ride and traction.

Shorter 20” sidewalls are a bit less forgiving at full psi on the roughest roads, lowering pressure helps. This Cooper AT3 XLT 285/65R20 was initially tested at 37 psi, but is at 30 psi in this photo. Dropping the extra 7 psi greatly improved the ride.

Both sets of Cooper AT3 XLT tires, and other sets of rubber in our test fleet, were swapped on-and-off our 2017 Ram regular cab flatbed for comparison. Both sets of Coopers also saw some miles on our 2014 Ram crew cab.

The 20s went on a 1,000-mile vacation with my wife and our little Welsh Terrier Elsie. That trip included over 100-miles of unpaved roads, that were occasionally rocky, snow-covered, or rough washboard. I intentionally gave the 20s more time and miles as I have much experience with Cooper 18s, including the previous AT3. As of this penning both sets have seen a combined 5,700 miles.

Remote northeastern Nevada camp, during a weeklong on- and off-pavement camping trip.

There are many potential variables to putting softer or firmer tires, and taller or shorter wheels on a chassis. If your suspension is stiff or the load is light, maybe a softer tire is better, as long as it’s not overly susceptible to punctures. Softer suspension and/or huge loads may benefit from a stouter design. The bullet points below share the pros and cons of using 20” verses 18” Cooper Discoverer AT3 XLT tires for big loads on heavy-duty trucks.

20” Tire & Wheel Positives

-Substantial improvement in lateral stability from reduced body-roll/sway when compared to 17” or 18” tires of similar construction. Feels like the improvement from adding a heavy-duty rear anti-sway bar compared to the factory bar.

-Steering feel is better, tighter, and more responsive with less delay.

-Slow, mountain turns and switchbacks with warning signs are less concerning.

-Freeway hop/bed-bounce caused by concrete freeway expansion joints is reduced.

My 140.5” wheelbase regular cab is stable and controlled over most surfaces, with inherently little freeway hop compared to the 149.5” wheelbase crew cab. Mounting the Cooper AT3 XLT 20” wheels on the crew cab both firmed-up the ride as we’d expect, and noticeably reduced freeway hop. 

An important safety consideration is that even if the chassis feels stable and composed enough to increase speed in certain situations, there is only so much available traction. When we use more of the total traction available, we are reducing our safety cushion, and putting more load on the tires, which causes wear and reduces longevity.

Does a 20” tire and wheel work best for your late-model truck? The answer may depend on your needs, preferences, and duty-cycle.
…or does an 18” tire and wheel work better for your application? You be the judge.

20” Tire & Wheel Negatives

-Noticeably less forgiving and rougher than the 18” tire over speed-bumps and on rocky roads at full street pressure. However, when the going gets tough, reducing psi benefits all tires and sizes tremendously. To get the same ride over rocky terrain, the 20” tire liked being about five psi lower than the 18”, particularly the fronts.

-Small bumps, cracks, or holes on the roadway are more noticeable, as the tire has less ability to absorb the imperfections. How noticeable depends on the tire construction, pressure, load, and surface. With my heavy Hallmark camper, 20″ Cooper AT3 XLT, which is admittedly a softer design, never felt harsh or uncomfortable on the pavement.


My 18” vs. 20” test driving quickly convinced me that with heavy loads a shorter and/or firmer sidewall that reduces some of the squishiness of a more flexible design was a positive. The biggest tradeoff is on the roughest, unpaved surfaces, which can be mitigated by adjusting tire psi and/or choosing a taller size.

With a given design like the Cooper AT3 XLT used for this comparison, the shorter 20” sidewall was also firmer and less flexible. However, if you change the tire construction characteristics, you have changed the suspension, and you have a new baseline.

Mounting the stoutly-constructed Cooper Discoverer S/T MAXX (or similar) with 3-ply sidewalls in an 18” size delivered better overall control than the softer 20” AT3 XLT. That is not a dig against the AT3, as it intentionally targets a different audience. Want or need another level of firm performance over an 18” Cooper Discoverer S/T MAXX? You can move up to one of the 20” S/T MAXX sizes. Modifications lead to modifications, and every change can affect something else.

Cooper S/T MAXX is a favorite heavy-duty hybrid/commercial traction design. Even a 1” wheel diameter difference is easy to see and feel. Both about 34” tall, 17” Power Wagon wheel (left), and forged Big Horn 18” wheel (right).

My travels include more off-pavement miles than many, so I am still evaluating where I want to make my long-term compromises, but like everyone, most of my driving is on good, paved roads. Currently I’m preferring 20″ tires for my flatbed camper outfit. My tire and suspension evaluations are continuing, and you can find additional information about these and other subjects here on RoadTraveler.net, or on Instagram, @RoadTraveler.

The sign says, “TRAILERS NOT ADVISED BEYOND HERE”. My camper is frequently off-pavement, but like nearly everyone, most miles are on blacktop.

Tell ’em you saw it on RoadTraveler.net

Become a RoadTraveler patron. Thanks!

James Langan

Copyright James Langan/RoadTraveler. All Rights Reserved


Cooper Tires

Hallmark Campers



Cooper Tires AT3 tire family

Cooper Tires’ Scott Jamieson, Director of Product Management, gives us the rundown on the new AT3 4S, LT, and XLT designs at the 2018 SEMA Show.

Copyright James Langan/RoadTraveler

All Rights Reserved.


Cooper Tires


Tread Matters: Tire Selection and Fuel Economy

2014 Ram mpg test platform.

Tread Matters: Tire Selection and Fuel Economy

Tires have been a popular subject in magazines for decades, and forums continually see new threads seeking information and expertise. This is partly because they are expensive. They can also provide dramatic style and performance improvements and are an easy upgrade. With so much talk, it is surprisingly difficult to get unbiased, detailed, and authoritative information.

Fuel economy is another perpetually popular topic. Since tire choice affects mpg, or so we have always believed, the subjects are intrinsically linked. Some folks don’t care about mpg, but many care a lot. Except for the purchase of a new(er) truck or major repairs, fuel is our biggest operational expense.

What if it was possible to improve your highway mpg by 5 or 10%? Not a possible increase from the latest magic program pushed by a snake-oil salesman, but simply by choosing a different tread design? An improvement that could be measured and verified, repeatedly, with real world testing, not just theory or laboratory results that are difficult, if not impossible, to replicate.

What Affects MPG?

In the enthusiast truck world it is commonly accepted that bigger rubber reduces miles-per-gallon. Maybe, but bigger is not specific, sometimes it means wider, taller, or both. Taller tires will increase the overall final-drive-ratio, which can help or hurt efficiency depending on the platform and usage.

Previous tests with my 3.42:1-geared, 2014 Ram/Cummins 2500 used for this article indicate that taller meats up to 35” helped economy, or at least hurt mpg less than one might expect when unloaded. Tradeoffs include less torque and slower acceleration from higher final-gearing, though current generation trucks make plenty of torque and horsepower for most reasonable loads. My sense is that stepping-up to 37s would require lower differential gears for optimal performance.

When folks upgrade their tires, particularly on a four-wheel-drive, they often switch to a higher-void pattern; sometimes the more aggressive tread is chosen simply for looks. Even if we don’t mind the road noise or faster wear of an aggressive pattern, how much fuel does looking cool consume if one rarely or never drives off-road? With multiple, simultaneous changes, it’s impossible to say what caused a reduction in fuel economy. Instead of belaboring what modifications can do to our trucks, or what affects what, I’ll briefly quote myself, “Modifications lead to modifications.”

Beefier tires might cost you more mpg than you think.
Three great tread choices depending on your priorities.

Controlling Variables With Cooper Discoverer Tires

With generous support from Cooper Tires, I performed a series of real-world tests to document how tread design (or pattern) or tread width impacts fuel economy. I invested a substantial amount of time and money to prove or disprove commonly accepted hearsay and to produce solid data I could not find anywhere. The pattern design tests are complete, and my procedures are detailed below together with the results in Table Two. The width results are concerning, or at least surprising, and additional work may be necessary to become comfortable with the facts.

Worth $11.
Love knowing what it really weighs.

The primary variable to be controlled for the design test was the size, but weights, odometer and speedometer error, wind, and temperatures were also logged. The bullet list below offers details.

  • Weather forecasts were monitored until several similar days were on the horizon. Because wind is common in Nevada, and typically increases with the afternoon temperatures, just one test was performed each morning, avoiding the higher winds and heat that would influence outcomes if I conducted multiple runs each day.
  • Three used sets of the same 29.8-pound, forged-aluminum (WBJ) Ram Bighorn 18” wheels were purchased from Craigslist, allowing all tires to remain mounted and balanced in case a test needed repeating.
  • Odometer error was measured for every design using mile-markers (MM) and GPS, as different treads in a certain size are not dimensionally identical. A single, constant-GPS distance was used for all mpg calculations. Road speed was monitored with GPS and corrected speedometer measurements.
  • To reduce the possibly of substantial inaccuracies during fueling, and to increase the validity of the data, the roundtrip route distance was 222.7-miles, over mostly level freeway.
  • Refueling was done at a particular pump, on the slowest fill rate to prevent foaming, and never topped-off. The freeway onramp is just one mile and three stoplights from the filling station.
  • Appropriate, not maximum, pressures were used for the modified but unloaded truck- 8,900-pound GVW.
  • The tailgate was up and the A/C was on.
  • Cruise control was used and only adjusted or turned-off briefly when absolutely necessary, and notes were logged regarding any irregularities. If an accident, construction, or other mishap would have caused stopping or a substantial speed adjustment for an extended distance, I would have aborted and repeated the test.
Same GPS distance used for all runs.
EVIC mpg info is often inaccurate, doing the math is better.

All-Terrain, Commercial Traction, Or Mud-Terrain?

When enthusiasts upgrade their rubber it’s common for choices to fit into one of three categories; all-terrain, commercial traction (hybrid), or mud-terrain. I chose the LT295/70R18E size, which is approximately 34” tall and 12” wide, with an impressive 4,080 pound capacity at 80 psi. Cooper offers three of their popular, yet distinctly different Discoverer patterns in this size: the Discoverer A/T3, Discoverer S/T MAXX, and Discoverer STT PRO.

Readers should remember than although every effort was made to limit variables, these were real-world tests using off-the-shelf products; some differences naturally exist. One easily overlooked fact is that tire compounds are proprietary, and each has its own special cocktail. Tread depth, and sidewall and tread plies also vary depending on the terrain and audience targeted. So the differences affecting performance and mpg are not just the visible patterns, but they include the compounds and the overall construction of each tire.

Cooper A/T3, S/T MAXX, and STT PRO designs.

Discoverer 295/70R18E Measurements

The differences between tires of a particular size are often small, though one should be careful when comparing those from different manufacturers and/or a vastly different pattern. Over the past decade I’ve evaluated several sets of Cooper-branded and Cooper-manufactured tires, and my measured values have repeatedly matched the published specifications. Occasional, slight variations appear to be from measuring tools, mounting on narrower rims, etc. Manufactures know precisely what they are producing; they want to be as accurate as possible. Careful measurements were made of each Cooper design, and the details are in Table One.

Reading forums leads me to believe that some consumers don’t measure accurately, and/or expect the on-vehicle dimensions to be identical as the wheel-mounted, off-vehicle measurements; these folks cry foul when they are not. That is silly, as the weight of the vehicle, psi, and wheel width all affect the on-vehicle stature, and this is something the manufactures have no control over.

If you read carefully, and do some math, you may notice that the measured weight of a solo tire, plus the 29.8-pound wheel, does not match the mounted data, there are a few extra pounds in the sums. I’ve seen this many times before, as measuring bare wheels is difficult, and generally I must hold them against my chest and subtract my body weight. The figures listed in table one are what my shop scale, a good bathroom scale, indicated, plenty accurate for weighing heavy auto parts. Emphasis should be placed on the mounted weights, as nobody drives on wheels without tires. The few pound difference between these designs is negligible on a heavy-duty truck with prodigious torque and weighing nearly 9,000 pounds.

All 295/70R18 tread designs were about 34.25” tall.
Straight edge, a rule, and a keen eye were used to record height and width.

Table One, 295/70R18 Measurements

Cooper Discoverer 295/70R18 A/T 3 S/T MAXX STT PRO
Weight (pounds) 60.2 66.4 68.8
Weight mounted (pounds) 92.0 97.4 99.8
Height unmounted 33 11/16” 33 13/16” 34”
Height mounted @60 34 4/16” 34 5/16” 34 5/16”
Tread width 9 7/16” 9 10/16” 10 3/16”
Tread depth 17/32” 18.5/32” 21/32”

Table Two, Tread Affecting MPG Test Data

Tread Matters MPG 295/70R18 A/T3 S/T MAXX STT PRO
Test GVW (pounds) 8,900 8,900 8,900
Tire PSI F/R 60/40 60/40 60/40
Date 9-28-16 9-27-16 9-29-16
Time 0832–1202 0859–1230 0837–1208
Temperatures F 54-59-72-70-76-72 52-58-67-73-75-78 58-60-56-69-79-76
Wind/Gusts 1/2-5/5-2/5-4/4-1/2 4/4-2/3-0/0-1/2-0/0 0/0-3/4-5/6-3/8-0/0
Odometer 27,241–27,459 26,950–27,168 27,503–27,721
Trip Odometer 217.6 217.7 217.5
Odo error % MM & GPS 2.24/2.34 2.17/2.29 2.28/2.39
GPS (miles) 222.7 222.7 222.7
MPH indicated 64 64 64
MPH GPS 65.0–65.5 65.0–65.5 65.0–65.5
RPM Tach/Edge Insight 1,700/1,677 1,700/1,680 1,700/1,677
Fuel used (gallons) 10.380 11.033 11.533
ECM indicated MPG 23.2 22.4 21.0
MPG calculated 21.45 20.18 19.30
The less aggressive Cooper A/T3 delivered much better mpg.

Tread Results Commentary

Choosing the Cooper S/T MAXX over the STT PRO mudder offers a 4.6% bump in fuel economy. Running the A/T3 instead of the S/T MAXX delivered a 6.3% increase. The leap from the STT PRO up to the A/T3 is 11.1%. Wow!

It’s impressive that a modified, heavy-duty, 4WD pickup with prodigious capabilities, weighing 8,900 pounds empty, with 34” x 12” tires mounted, can still reach or exceed 20 mpg during highway travel. Obviously most driving involves at least a few and stops and starts, but these repeatable tests demonstrate what is possible if speed and idling are minimized. If I picked the Discoverer A/T3, it appears that long distance highway runs, even with a couple pit stops, could top 20 mpg.

If one needs the extra grip offered by the STT PRO or S/T MAXX, choosing the A/T3 all-terrain might not be an acceptable tradeoff. Yet, if one is so inclined and has the space, these numbers seem to reinforce the practice of having two sets of tires and wheels. Whether they are all-terrains and mudders for your truck, or highway and winter rubber for your car, strong arguments can be made about picking the right tool for the job. We don’t wear flip-flops to go mountain climbing, and our clodhoppers are out of place in a gymnasium.

Off-highway traction is great with a mudder, but you will pay at the pump.

Does Width Matter?

The initial primary platform for measuring how tread width affects mpg was my modified, heavy, and low-geared ‘06 V8 4Runner, because I already had one of two desirable sizes. One might think the results would be relevant for most light-truck platforms. The conditions and procedures were the same as those for the different tread patterns.

I used Cooper’s S/T MAXX in 255/80R17, and 285/75R16, both 33” tall, but the 285s are substantially wider. The 255s are about 10” wide and the 285s about 11.5”; conventional wisdom says the 285s would consume about 1–2 mpg. Without creating another table, the short story is that theses tests delivered ambiguous results, there was very little difference. I was shocked! Followup runs might indicate these results were a fluke, but there were no obviously problems or procedure inconsistencies. The narrower 255s delivered 18.34 mpg, and the 285s 18.22 mpg.

I thought wider treads consumed more fuel, not so sure anymore.
On the car or dismounted, the 285s are much wider than 255s.

Ram Width Comparison

Two sets of tires and wheels for my ‘14 Ram partially met the width criteria, meaning they were very similar height with the identical tread pattern, yet the width difference was small. I had them, so test I did, using the same parameters, during the same week, weather conditions, etc.

One set were the 295/70R18 S/T MAXX in table one. The other were 285/75R17 S/T MAXX mounted on 2015 (WFV) forged aluminum Power Wagon wheels, which weigh 28.6 pounds each. These 285mm-wide Discoverers are also 34” tall, but just fractionally shorter than the 295s. The mounted, measured tread width difference between these two sets is only about 3/8”.

The seventeens were evaluated at the end of six consecutive days of testing, and the weather started to change, with 22 mph winds near the end of this last trip. This was noteworthy, but I’d argue that there was not enough wind during most the run to impact the outcome. The data appear to support that opinion. For the slight difference in width, the results appear appropriate. There simply was not enough difference to influence economy, 20.18 vs. 20.22 mpg. I call that a draw.

295/70R18 vs. 285/75R17. Not a huge width difference but still easy to see.
Forged 17” Power Wagon wheel on the left, forged 18” Bighorn wheel the right.

Table Three, 285/75R17 Measurements

Cooper Discoverer 285/75R17 S/T MAXX
Weight 64
Weight mounted lb. 93.4
Height unmounted 33 11/16”
Height mounted @60 34”
Tread width 9 1/4”
Tread depth 18.5/32”

Table Four, Ram Width Matters 285/75R17

Width Matters MPG S/T MAXX 285/75R17
Test GVW lb. 8,900
Tire PSI F/R 60/40
Date 9-30-16
Time 0759–1130
Temperatures F 55-59-69-71-73
Wind/Gusts 0/0-3/4-5/5-5/14-22/22
Odometer 27,780–28,000
Trip Odometer 219.0
Odo error % MM & GPS 1.57/1.68
GPS miles 222.7
MPH indicated 64
MPH GPS 65.2–65.7
RPM Tach/Edge Insight 1,700/1,690
Fuel gallons used 11.011
ECM indicated MPG 20.9
MPG calculated 20.22
Good traction with a tolerable mpg penalty, I prefer a hybrid/commercial traction tires like the Discoverer S/T MAXX.

The Ram results were not a big surprise. The lack of separation between the 4Runner’s width test mpg numbers, and to a lessor extent the Ram’s, have me questioning how much tread width alone impacts fuel economy. Much taller tires, with the corresponding overall gearing changes, combined with more aggressive tread patterns may be the main story behind fuel economy losses when fitting aftermarket rubber. Sometimes testing answers questions.

James Langan

Copyright James Langan/RoadTraveler/Turbo Diesel Register. All Rights Reserved.

 A version of this article was also published in the Turbo Diesel Register magazine.


Cooper Tires: coopertire.com


Cooper Discoverer A/T3


Testing & Talkin’ Tires

October 6, 2016

Even casual readers of this site will notice that I’m a light-truck tire aficionado; there are many posts about rubber for light-trucks. My personal obsession aside, there are powerful reasons tires are such a popular topic for both writers and enthusiasts nearly everywhere we gather. Mounting new meats is one of the easiest and most dramatic performance and/or appearance modifications owners can make to their trucks. Replacing worn rubber with new, even the same pattern, can greatly improve safety and traction. If you have any doubts, watch this Tire Rack video regarding tread depth and stopping distances on wet roads: tirerack.com/videos/index.jsp?video=5&tab=tires

Looking through a historical lens, modern tires are generally excellent, with unsurpassed designs and sizing options, and they are a good value. Yet value doesn’t mean inexpensive, and depending on the size and performance category, a new set of shoes for your truck can easily top $1000. This substantial outlay leads to questions and much research for many buyers.

Still Plays With TIRES means frequent trips to tire stores with a few shoes and insoles. This is a moderate load, sometimes I need a bigger trailer.
Still Plays With TIRES means frequent trips to tire stores with a few shoes and insoles. This is a moderate load, sometimes I need a bigger trailer.

Journalism’s Dirty Tire Secret

If you read truck tire reviews critically, you may realize that many involve very few miles of use before the evaluation is penned, often as little as a few hundred miles. Reasons for this include the long lead-time for print periodicals, editors’ desire to publish something as quickly as possible, and sometimes a little pressure from the manufacturer or advertising agency folks. Writers sometimes mount new tread and take them on a little excursion, writing much about the adventure and some about the tires, then use this one experience as the appetizer, main course, and dessert. Meh.

Another favorite is the manufacturer’s initial ride-and-drive test at a testing facility or track. When possible I happily attend and enjoy such events, but they are mostly a good introduction. If they’re not followed with a longer, personal-use test, they often don’t tell the complete story.

When one brand redesigned their super-popular all-terrain pattern two years ago, they hosted journalists in Baja where the test vehicles were race buggies and Ford Raptors. I have no doubt that the conditions and obstacles were gnarly, and I’m not saying the product isn’t good. But how does one test a tire’s performance on an unfamiliar chassis, particularly on a race buggy or (factory) desert-prerunner truck? Where is the baseline? Are the tires being tested, or is the complete chassis? Would these highly-capable vehicles perform impressively if another tire brand or design was mounted? Surely.

Hopefully readers can benefit from my continuous evaluations. Instead of buying a new set every few years like many consumers, some running the same or similar treads repeatedly, I typically test a few sets each year. My personal experience and database over the past two decades is quite large, and includes aggressive mud tires, tame all-terrains, and many in-between. Although I swap tread often, I dismount them from wheels infrequently. At any given time I have several sets of tires on OE wheels, currently six that fit my 2014 Ram 2500, and keep notes on the dates, miles, performance, and wear. Some I buy, and some are supplied by manufacturers for review. Just this week I sold two older sets, one Ram and one Toyota, and bought a new set for my 2500. Some get more miles than others, depending on my needs and preferences, the physical size or fit, and how well they mesh with current objectives, but all receive thousands not hundreds of miles. Several years ago a teasing friend dubbed me “the Imelda Marcos of tires.” What can I say, if the shoe fits….

Starting lineup. There are few truck parts (any?) I like more than a fresh set of rubber.
There are few truck parts (any?) I like more than a fresh set of rubber.

Cooper Discoverer A/T3

Over the past several years Cooper Tire and Rubber—which is still a U.S.-based company and manufacturer—revamped their light-truck line. The 5-rib all-terrain Cooper Discover A/T3 is a natural choice for someone wanting better traction in more varied conditions than a highway tire (HT) offers, but something quieter, smoother and softer than a commercial traction pattern like Cooper’s S/T MAXX (which I’ve run on my 4Runner for a few years). The performance improvement over an HT can be substantial in inclement weather, including something as common as a hard rain, but the differences can be even more dramatic with a little snow, slush, or ice covering the roadway.


Because the A/T3 is their flagship all-terrain tire there are an impressive 56 sizes. The outer rib’s open lugs allow liquid and debris to escape better than highway designs, as do the circumferential voids in the center. The silica-based compound improves wet traction and on-highway handling, provides cut and chip resistance on rough terrain, and reduces rolling resistance. Lateral groove protectors reduce stone retention and drilling, and the broken center rib is designed to improve soft surface traction. It is M+S rated, and has a 55,000 mile tread wear warranty.

There will always be a place in my heart and space in my garage for high-void traction tires, though maturing has made me increasingly less fond of louder designs when they are not necessary. The A/T3 is pleasant, barely audible to my ears, and notably quieter than the similar but slightly higher-void 5-rib Toyo A/T II tested on my Ram for 8,000 miles. (The Toyos averaged 1/32 of wear for every 2,100 miles, with frequent rotations, and were removed to mount the A/T3s.)

Comparing Cooper’s high-void, 295/70R18E STT PRO mudder to the the 285/75R18E A/T3. Both sizes support 4,080# each at 80 psi.
Comparing Cooper’s high-void, 295/70R18E STT PRO mudder to the the 285/75R18E A/T3. Both sizes support 4,080# each at 80 psi.

Again I chose the fantastic, niche, LT285/75R18 size. Cooper is one of a handful of companies making this approximately 35×11.50 inch size, tall but not overly wide. These Coopers are 34.84-inches tall, with 17/32 of tread depth measuring 8.9-inches wide, and weighing 58.4-pounds solo and 90 when mated to Ram Big Horn WBJ forged aluminum wheels. They fit perfectly on the stock 8-inch wheels, and like any pattern in this size, will support a massive 4,080 pounds at 80 psi. Loaded to the Ram’s GVWR, with 60 psi in front and 80 psi in back, the rear differential ground clearance is 8 3/4 inches.

Balancing Act 

Using my favorite local Discount Tire store the Coopers were dynamically (dual-plane) balanced. As always Centramatics balancers work in the background, adjusting to any irregularities on-the-fly. The A/T3s took very little wheel weight to balance, and they have remained smooth at all speeds, legal and above.

Inside              Outside

#1 3.00            0.25

#2 1.75            1.75

#3 1.50            3.00

#4 1.25             3.00

The LT285/75R18E Discoverer A/T3 starts with 17/32” of tread.
The LT285/75R18E Discoverer A/T3 starts with 17/32 of tread.

Ride quality is smooth and compliant; the traditional construction 2-ply sidewall is not stiff, and helps absorb impacts, even at full pressure under a maximum load. The generous and squiggly shape of the siping helps grip, and is surely behind some of the excellent winter traction endorsements I’ve read on snow plowing sites (my A/T3s have not seen much wet yet). Straight-line tracking is good as one would expect from a 5-rib all-terrain/all-season design, as is steering response. When conditions are right my truck will drive straight for 10 seconds or more with no input. It’s too early to report on wear, but after the first 2,500 miles, it looks mileage will be similar to the Toyo A/T II tires mentioned above.

The A/T3 doesn’t feature or need sidewall tread for its target market.
The A/T3 doesn’t feature or need sidewall tread for its target market.

Supporting Documentation

Confidence in my prose is important, but I enjoy sharing others’ views when it helps make a point. Before accepting the Senior Editor post at OutdoorX4 magazine, I was a technical editor at Overland Journal (OJ) for a few years. For the Summer 2014 issue, OJ conducted a comprehensive, seven tread, all-terrain comparison which was later published online, and can be read at: expeditionportal.com/where-the-rubber-meets-the-road. The article is a good read for traction tire enthusiasts. The short version is that the Cooper Discoverer A/T3 won both prizes after all the tests were completed: the “Value Award” and “Editor’s Choice”.

For a less analytical but impressive amateur review, this YouTube link gives a snapshot of the A/T3’s winter performance potential. A competitor’s design with plenty of tread remaining cannot start up a snowy incline in 2WD, but with Cooper A/T3s mounted, the truck moves forward.


If you are in the market for a traditional 5-rib all-terrain, but with an updated design and reputation for superior traction, consider the Cooper Discoverer A/T3.


Cooper Tires: coopertire.com, 800-854-6288

A version of this article was published in Issue 93 of the Turbo Diesel Register magazinein my Still Plays With Trucks column.

Copyright James Langan/RoadTraveler.net