The custom auxiliary fuel tank project for our 2017 Ram/Cummins 2500 is underway! June 2019 I added two additional aluminum toolboxes aft-axle on my Hallmark Nevada Hillsboro flatbed camper outfit. My idea was to eventually remove one of the large, 30-inch-long front boxes and replace it with a fuel tank, preferably a commercially available aluminum saddle tank for a medium-duty truck.
The tank project was moved to the back burner and a year passed quickly. For a while I considered a much smaller 10 or 12-gallon tank, hoping to fit one under the chassis between the frame rails, and in front of the spare tire. Just 100 miles of additional range would be a game changer during long highway trips.
Evan a commercially available tank would have required custom mounts and modifications, and surely some compromises. After discussing the project with Chris at High Sierra Fabrication several weeks ago, we decided on a custom tank for my original under-bed location. The goal was to fabricate a tank that looks extremely similar to the toolbox that was removed, and the matching 30-inch box still on the passenger-side.
Custom costs more, takes time, and is never snag-free, but if you have the right business and people involved, the results can be fantastic.
A beautiful tank was constructed using 3/16-inch diamond plate aluminum. Then High Sierra Fabrication proceeded to drill and cut holes in the box, weld-in fittings, a sump, and clean-out port, etc. We are getting close, but still working on it.
After High Sierra Fabrication completes most of installation, I will still need to plumb and wire the external pump to transfer the auxiliary fuel into the 2017 Ram’s factory tank.
Attempted to establish a baseline for fuel economy/mpg at 65 mph, with the 35.4″-tall, 295/65R20 Cooper STT PRO tires under a heavy Hallmark flatbed truck camper outfit. There was too much wind for this to be a true baseline, but efficiency was still good for this not-aerodynamic configuration; just under 13 mpg. I probably will revisit this in the future.
A recent Nevada Highway 95 trip from Northern Nevada to Southern Nevada, and back, a very familiar route, produced some interesting data. People often make contrary statements about shared fuel economy information, generally well intentioned but not always accurate, including:
1) My similar truck/camper does much better (or worse). 2) An aftermarket engine tune would produce superior results (doubtful but possible). I could write several such hearsay proclamations.
Let us remember that similar is not the same, andthis includes engine tunes, programming, and power ratings. Modifying engine performance is not an option as long as I want to keep my emissions equipment on my truck and continue registering it in the zip code where I live. However, my 350 horsepower, G56 manual truck has plenty of torque and power, and if I wanted more I’d likely regear the differentials.
Now, let’s review some fuel economy 101.
Everything that is different can matter, including:
-Engine output (also brand, configuration, etc.)
-Weight (less critical than many think, depending on the outfit)
However, the two big ones often not fully appreciated are:
–Engine speed: pumping losses
–Vehicle speed: aerodynamic losses
Engine pumping losses are of minimal concern for this rig. The 2017 Ram/Cummins has the tall factory 3.42:1 gears, a 6-speed manual transmission, and runs 35-inch tires, resulting in low rpm in overdrive.
The aerodynamics are horrible. This is a heavy-duty truck with a big winch bumper/grille guard, sliders, and a flatbed with mud flaps; these are important details. However, the frontal area and drag coefficient of the truck camper (and truck) are a huge factors.
It is possible to get similar (or better) mpg with a heavier truck and trailer combination that is more aerodynamic; that’s how critical wind load and speed are.
Fuel Economy Data
The test below was informal, if you want something more detailed, with additional focus on controlling variables, you’ll likely enjoy this post: Tread Matters
Southbound: Fernley to Beatty, Nevada
Vehicle: 2017 Ram 2500 Cummins/6-speed manual
Date: March 2, 2019
Distance: 295.6 miles (odometer corrected)
Cruising speed, where legal: 65 mph
Approx. gross weight: 11,500 pounds
Northbound: Las Vegas to Hawthorne, Nevada
Vehicle: 2017 Ram 2500 Cummins/6-speed manual
Date: March 4, 2019
Distance: 302.0 miles (odometer corrected)
Cruising speed, where legal: 63 mph
Approx. gross weight: 11,500 pounds
Speed Eats MPG
Because the terrain and conditions over Nevada Hiighway 95 have repeatedly given better fuel economy going southbound than northbound, I intentionally reduced my speed going north to try to limit the difference. It worked, two mph made a difference. My educated estimate is that at 65 mph, fuel economy would have been in the high twelves on the trip north, and going as fast a 70 mph would have droped mpg into the mid to low twelves.
Want a quick example of how much speed and conditions matter? The same outfit when it weighed less and was slightly more aerodynamic with the stock front bumper, cruising almost completely at a constant 75 mph on Interstate 80 across Northern Nevada, Elko to Reno, includingmaintaining 75 mph up and over mountain passes, and a bit of off-highway travel to camp, yielded 11.57 mpg over a distance of 300.4 miles. Considering the use and conditions, that was an appropriate result, inline what was requested from the chassis.
Copyright James Langan/RoadTraveler All Rights Reserved
23.21 MPG from a 2017 Ram/Cummins. 65-mph highway baseline test, Sept. 5, 2017
222.7 miles, 9.593 gallons
65-mph freeway/highway test
Tires: F 60 R 40 cold
Regular cab, long bed, 7,400-lb. GVW
Newer diesel trucks are not as efficient (due to emissions systems) as previous generations? Maybe. Probably. Though it depends on the exact year, model, and version. The true potential is better than some think, and the conditions, load, and nearly everything else matters.
Tests must be repeatable and confirmable. The ECM was slightly pessimistic here, and the odometer a little fast with the stock tires.
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.
Recently a friend emailed this miles-per-gallon question:
I’m scratching my head here on fuel and mileage numbers. If I do the math for gallons of fuel consumed (fuel filter life % gauge) and odo I get about 16 mpg.
But the Ram’s EVIC dash display says 11 MPG.
With 4.30:1 gears and 37-inch tires, I know ALL the numbers are inaccurate.
How do I figure this out?
I’d be very surprised if you are able to routinely obtain 16 mpg with your Ram/Four Wheel Camper setup, unless you are driving 55 mph. I never trust dash displays. They are almost always overly optimistic, though in your case because of the tall, 37-inch tires, it may actually be a little pessimistic. My built, 2006 4Runner mpg display is slightly low most of the time because the car travels further on taller tires than the ECM calculates.
The fuel filter life gauge is not a good source for mpg data, I think it’s really only useful for when to change the fuel filters. I’ve done similar calculations using the oil life percentage numbers (it probably uses the same ECM algorithm) after doing oil changes. The numbers literally do not add-up to the actual odometer distance traveled (neither indicated miles or the known and measured inaccuracy). To state the obvious, the only way to get accurate fuel economy readings is to have good numbers for the math; how many miles vs. how many gallons at fill-up.
I don’t think your gearing change matters. Most late-model vehicles calculate the speed from wheel sensors… as long as that’s the case on the new Ram (easy to test) then it is only the tire diameter that matters. I’ve tested many sets of 33, 34, and 35-inch tires on my 2014, but no 37s yet. My guess is that your odometer is about 8-10% slow, compared to the actual miles you are traveling.
My method of testing tire-induced odometer error is to compare actual miles traveled to odometer readings. California doesn’t use highway mile-markers most places anymore, but Nevada does. With two columns on paper, I reset my trip odometer at a mile-marker, and then log the indicated and mile-marker distances. Data gathered over more miles will be more helpful. For example, a 10-20 mile test can be better than nothing, but the initial error percentage will drop if you make a longer run, like 50-100 miles.
If you’re on a long trip you can use indicated GPS miles instead of mile-markers. Recently I compared GPS to mile-marker data over a 70-mile route, and was surprised that they were just slightly different.
Copyright James Langan/RoadTraveler. All Rights Reserved.