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.”
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.
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.
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.
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.
Table One, 295/70R18 Measurements
Cooper Discoverer 295/70R18
Weight mounted (pounds)
Height mounted @60
Table Two, Tread Affecting MPG Test Data
Tread Matters MPG 295/70R18
Test GVW (pounds)
Tire PSI F/R
Odo error % MM & GPS
RPM Tach/Edge Insight
Fuel used (gallons)
ECM indicated 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.
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.
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.
Table Three, 285/75R17 Measurements
Cooper Discoverer 285/75R17
Weight mounted lb.
Height mounted @60
Table Four, Ram Width Matters 285/75R17
Width Matters MPG
S/T MAXX 285/75R17
Test GVW lb.
Tire PSI F/R
Odo error % MM & GPS
RPM Tach/Edge Insight
Fuel gallons used
ECM indicated MPG
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.
Copyright James Langan/RoadTraveler/Turbo Diesel Register. All Rights Reserved.
A version of this article was also published in the Turbo Diesel Register magazine.
Most light-truck tires are welcome in my garage, from tame all-terrains to the impressively streetable modern mudder. However, I have a strong preference for rubber that fit neither category, those that intentionally blur the lines of distinction, finding their own focus. Known by their traditional name, commercial traction tires, or aggressive all-terrains, hybrid, or the newer slang moniker, tweener (in-between), the design goal is similar.
Commercial traction tires are not new, they have been produced for decades, though the choices were fewer and they rarely received much marketing budget. Still not necessarily the beneficiary of the biggest advertising campaigns, depending on the brand, the performance advantages of modern hybrid treads have won-over many enthusiasts as a practical choice with fewer compromises. This segment of the market demands good grip on multiple surfaces, load-carrying capability, and puncture resistance.
Notably better in sloppy stuff than a typical all-terrain, with less noise and superior versatility than muds, there is much to like. Some have more sipes and the 3-peak/snowflake winter rating, while many don’t, but still perform well in the wet stuff. Nearly all have prodigious tread depth and void—particularly at the outer lugs—that broadcast their ability to absorb and fling muck when required. Mud-terrains are often described as 20/80-designs (20% road 80% dirt/mud), but commercial tractions are generally 60/40, 50/50, or 40/60, depending on their characteristics.
Mastercraft Courser CXT
In May 2016, Mastercraft Tires introduced their new light-truck (LT) commercial traction tire, the Courser CXT. It’s a mid-void, 4-rib that replaces the Courser C/T (C/T = commercial traction). Mastercraft is a subsidiary of the Cooper Tire & Rubber Company, one of the few remaining American tire companies and manufacturers. Before sharing my observations and insights, what Mastercraft says about their product is quoted below in orange:
The Courser CXT was designed as a premium light truck commercial traction tire that provides trusted all-terrain performance with enhanced off-road durability. The CXT features variable full depth siping and a silica rich tread compound for enhanced wet and winter traction. The large tread element and blocky design help to resist abnormal wear while enhancing tread stability and durability.
Large Surface Area Tread Blocks
Provide increased grip on and off-road while improving wear performance.
Optimized Void-to-Rubber Ratio
The amount of rubber on the road is optimized to provide rough terrain traction and enhance on highway driving comfort and feel.
Enhanced Upper Sidewall Design
The shoulder design increases off-road traction with side traction blocks and the circumferential raised rubber feature protects against sidewall abrasion and impacts.
Large Shoulder Scallops
The scallops provide a “mud-scoop” effect for dependable off-road traction while giving the CXT a more aggressive look, to enhance the appearance of almost any light truck vehicle.
The CXT is offered in 29 sizes, starting with the oldie-but-goodie 31×10.50R15LT, up to the 35×12.50R20LT. The size breakdown includes three 15-inch (all load-range-C), nine 16-inch, nine 17-inch, five 18-inch, and three 20-inch sizes. All sport a substantial 18.5/32-inches of tread depth, offering potentially more grip and longevity than others that start with less. In addition to the M+S rating, the CXT can be studded.
For those familiar with Cooper’s other LT designs, it is easy to assume that the CXT is simply a different tread slapped onto their extremely popular and capable, Discoverer S/T MAXX casing. Not so, they are different tires, both tread and carcass. Yet, many considering the CXT will likely also consider the S/T MAXX.
Mastercraft CXT vs. Cooper S/T MAXX—Two Primary Differences
The Discoverer S/T MAXX employs Cooper’s Armor-Tek3 carcass, a 3-ply sidewall, whereas the Courser CXT uses a 2-ply design. There are pluses and minus to both depending on one’s needs; 3-ply sidewalls are generally more rugged and stiffer, where a 2-ply may flex better, ride softer, and weigh slightly less.
The S/T MAXX is optimized for severe cut and chip resistance. When the MAXX was added to Cooper’s light-truck line it’s closest sibling was the S/T (no MAXX), which was/is not nearly as cut and chip resistant. The Discoverer S/T is also a straight 4-rib, where the MAXX’s center alternates between four and five.
The CXT has extra silica for additional wet traction. The slightly higher-void of the 4-rib CXT is visually similar to the older Cooper S/T, but the CXT has deeper scalloped outer lugs, plus beefy upper-sidewall (shoulder) tread that the older S/T does not.
More or slightly less void, 2-ply or 3-ply sidewalls, increased wet traction potential or optimized cut and chip resistance…only you can decide.
Cult Of The 255 / The Third 255/80R17
Several of the 29 Mastercraft Courser CXT sizes could fit one of my vehicles, and I was tempted to pick a larger size. However, for nearly two decades I’ve run and been a fan of moderate width tires, chiefly the 255/85R16, and for a few years its 17-inch brother, the 255/80R17. Mastercraft makes the CXT in both of these sizes, and 255/8x aficionados are surely rejoicing!
One challenge for those wanting to move to the 255/85 size is the lack of treads with less void; many current 255/85R16 offerings are mud-terrains. The 255/85 has become a niche choice, with few newer trucks using 16-inch wheels. The 255/80R17 is even more specialized. Mastercraft’s introduction of the CXT raises the total number of tires offered in this size to three. Cooper makes two of them; BF Goodrich’s mudder is their only competition.
Height, Weight, Width
A super-clean set of fourth-generation Ram 17×8-inch WFK forged-aluminum wheels were purchased from a Craigslist seller, each weighing just 21.8 pounds with the hubcap. Unmounted, a 255/80R17 CXT registers 55.2 pounds on my shop scale (the same size S/T MAXX is 58 lb.), and once mated to a WFK wheel the combination measured 77.2 pounds. Inflated to the maximum 80 psi, the overall height was 32 15/16-inches, with 8-inches of tread.
It’s noteworthy that I’ve repeatedly found published specifications for Cooper-manufactured tires to be accurate. For this tire and size, on a 7-inch wheel, Mastercraft lists overall diameter of 33.15-inches, and tread width of 8.07-inches. Acknowledging that manufacturers’ measuring tools are likely more accurate than my straightedge and yardstick method, and the 255/80 CXT was mounted on a wider wheel, my measurements were still within nearly two-tenths. For the curious, the unmounted height was almost a half-inch shorter, but focusing on unmounted diameter is pointless: tires are not used without wheels and compressed air.
Mounting And Balancing
Manufacturing tolerances, weight, width, construction, and the wheel employed all affect how easily and well an assembly is balanced. In general, smaller and lighter equals easier to true. Using the static, single-plane method, the ounces of wheel weight required were:
The tires were immediately put to work supporting a maximum load on a built Ram that typically lives at its 10,000-pound Gross Vehicle Weight Rating (GVWR), carrying a Hallmark Milner camper, tools, and other supplies. With the fronts at 60 psi and the rears at 80 psi, the ride was neither mushy nor harsh. Over a familiar section of freeway where expansion joints can induce freeway hop (generally not a problem on this truck) the CXTs exhibited no such tendency. Steering response was excellent, a common trait with narrower tires, as they take less energy, time, and effort to change direction. Even at paralegal speeds, balance didn’t change and no bad-vibrations were felt.
Initial impressions were that noise is slightly more than the popular Cooper S/T MAXX, which is an impressively quiet design. This is not surprising as the CXT has a higher-void, 4-rib pattern instead of the MAXX 4/5-rib. The volume and deeper tone is not annoying or loud, and what I expected; both are certainly much quieter than any mud-terrain. The CXT sounds similar but slightly quieter than the older CooperDiscovererS/T (not to be confused with the S/T MAXX, STT or STT PRO).
Function is more important than form, but many like their 4WDs to look tough. Before receiving this set of Mastercrafts I’d not seen the tire in person, just the few marketing shots online. There were no substantive professional reviews or user reports. This article still may be the first. The outer lug scallops were a pleasant surprise, and the sidewall shoulder tread was beefier than I expected. Pretty sexy, in a nice, girl-next-door way.
Notes On Tracking
When changing to a different tread, size, and/or wheel, there is a possibility that your vehicle may need a custom alignment to match the new combination to the chassis. Some folks are willing to ignore a little drift (or pull) right or left, where others find any drift unacceptable. Some tires have a well-deserved reputation for directing vehicles to the shoulder or median, but different trucks and roads can cause different behaviors.
If your truck has an independent-front-suspension (IFS), adjusting the caster (and to a lesser extent camber) to help it track straight should be easy for a good alignment shop willing to make custom adjustments. Be willing to pay more. Finding such an establishment with a skilled technician may be challenging. Many places that should know better still want to use the factory geometry for modified rigs when different settings would fix or dramatically improve drivability.
Picking My Own Line
The 2014 Ram initial CXT test platform has a Specialty Products Company (SPC) 1.5-degree offset ball joint at the right-front, installed after only 1,500 miles to counteract the characteristic right-pull of many Ram trucks and/or some tires. Before any modifications, still running the stock Firestone highway treads, this truck drifted right and would head for the shoulder quite rapidly if the steering wheel was released, typically in six seconds or less. Unacceptable.
The SPC offset ball joint increased caster angle on the right, effectively directing the truck left helping the chassis drive straight without input from the driver to correct the right drift. With such an aggressive geometry modification there is always the possibility, even likelihood, that the truck will track left with some tires or under certain circumstances, including differing road crown. This was a compromise I was willing to live with, but it’s not for everyone. Swapping ball joints is not a trivial affair on a live-axle truck.
With the CXTs mounted, this truck has a slight tendency to go left, depending on the roadway. However, three “look mom, no-hands” tests during the first 100 miles, under suboptimal windy freeway conditions, achieved 12.06, 11.90, and 12.26 seconds before semi-autonomous driving had to be curtailed to prevent the truck from changing lanes. These are good numbers, but not surprising as narrower treads generally track (much) better than wide ones. This also means I’d be perfectly happy to run these on long road trips. The stars were aligned during another test on Interstate 5 in California where I clocked 25 seconds of straight tracking. A buddy’s Dodge that drifts right with most tires, still did with the CXTs mounted . Your truck may vary; adjust as needed.
All thirty-threes are not created equal, because they’re not all exactly thirty-three inches tall. The very common 285s are typically 32.8-inches tall while 255/85R16s are almost always 33.3+ inches; the Toyo M/T is 33.5-inches! Removing the Dick Cepek F-C II 17-inch 285s, slapping-on the 18-inch Cooper S/T MAXXs, and taking a few measurements showed that the taller 275/70R18 MAXXs were good for 3/16″ of additional clearance under the rear differential. Nice.
My initial S/T MAXX drive was on a test loop I typically travel after any tire, wheel, air pressure, alignment, or other drivability changes are implemented. It’s several miles long and includes city streets, rural highway, interstate freeway, and sometimes just a minuscule amount of dirt for photos. The route and terrain are familiar, as is the vehicle platform, what’s being evaluated is the change, in this case, tires.
What noise? Articulating tire noise can be a challenge, as vehicles have different levels of insulation and sound deadening, and drivers have their biases and tolerance or lack thereof for noise. With several back-to-back tire tests under my belt the S/T MAXX is clearly one of the quietest traction tires I’ve ever had the pleasure to drive on. The volume and tone is similar to my beloved Dick Cepek F-C II, but a bit quieter, and the MAXX is certainly quieter than the standard Cooper S/T. Comparing the S/T MAXX to the super popular BF Goodrich A/T is tough. While I currently have a set of used BFG A/Ts, I have few personal miles on them and I’m not a BFG A/T fan. I’ll go out on a strong limb and say the new S/T MAXXs are louder than the BFG A/T, but not much. For a tire that offers as much traction and void (much more than the BF Goodrich A/T) the Cooper S/T MAXX is an amazingly quiet tire and well worth the little noise it makes.
As noted in my introduction, the ArmorTek3® carcass construction and 3-ply sidewalls of the S/T MAXX are stiff. This can be great if this is a feature you are looking for, or a negative if you prefer a more compliant ride and/or a lighter tire. (If you are new to this blog or subject, read Wheels, Tires, and Sidewalls from Jan. 30, 2012.) In addition to the intentionally rugged design of the S/T MAXX, using 18-inch wheels with only a 33-inch tire adds to the lack of sidewall flex.
One of my strong dislikes for tall wheels on light-trucks is manifested by this situation: a relatively tall tire (33+), with a relatively short sidewall doesn’t offer the same off-highway flex and performance available from a taller sidewall. The same height tire on a 16-inch rim would yield an additional inch of sidewall to drive and flex on; this is not a small difference. If you already use and like the firmness of a load-range E sidewalls then the stiffness of the S/T MAXX (similar to the Toyo M/T) may not bother you, particularly if you don’t have to run it on a tall wheel. However, if you prefer a more compliant ride or a lighter tire you might reconsider the MAXX. What do you want, and what do you need for your application?
Pulling To The Right
Several years ago I rarely experienced tire pull problems with my trucks, however they are all live-axle 4WDs. Starting with my 2006 V8 4Runner and now continuing with this Tundra, I’ve experienced a pull (or drift) to the right with certain tires but not with others. Both of these IFS four-wheel-drives have alignment settings and geometry that are equal to or better than when new.
Aftermarket upper-control-arms are employed and expertly adjusted, though invariably some tires cause a very noticeable drift or pull to the right. I’ve spent countless hours and dollars to try and understand, identify, and combat this situation on these Toyota 4WDs. In short, my work involved many trial-and-error alignment adjustments, tire & wheels swaps, rotations, and tire pressure changes, all of which did not correct the problem. Some tires would still pull right, some terribly so.
How bad is the pull or drift? On a straight section of freeway with little road crown, at 65 miles-per-hour, releasing the steering wheel will result in crossing the right lane-line in about 3-seconds. Three seconds! I’m a strong advocate of keeping one, if not two hands on the wheel, and paying attention to the task at hand when driving. But I also like my vehicles to have very neutral handling, heading straight down the road unless instructed otherwise. Modified or not, I require my vehicles to drive almost perfectly on-highway. For comparison, depending on the road, wind, and other variables, with the same alignment settings Dick Cepek F-C II tires would continue straight for 8–10 seconds with no-hands.
Some readers may observe that the wheels are not identical. OE 17-inch forged TRD wheels are being used for the F-C II, while the MAXXs are mounted on 18-inch SR5 wheels, and there is probably a slight difference in backspacing. Is that part of the problem? I doubt it, though I did consider of this difference, and have a plan.
After an interlude, there will be more Cooper S/T MAXX commentary in the future—first we have to drive there.