37s giving 2.5 MPG better than stock?

[Zachanadandy]

I know that it goes against the grain of what would be expected. I also know that not everyone will see the same results with some not even getting close. In reading the post you will see the same Gladiator but with the previous owner averaged nearly 10 mpg less. I have exceeded the mpg posted by the EPA on every vehicle that I even owned with the exception of a BMW X5 with the M Performance Package and I struggled just to reach it. On the final trip to trade it in I averaged 63 mph. My average RPM was around 1750. I had as many downhill grades as uphills. I did not use AC ( April). I drove with the instant mpg display on and tried my best to obtain the best fuel economy. That is what I got. On the way back with the Max Tow on my first tank my average speed was closer to 72 mph. My average rpm was around 2100. I only got 21 mpg. The true difference in tire size was about 5 inches in diameter/height. The wheel/tire weight was around 112lbs vs maybe 70 on the Max Tow. I averaged 23.2 up to around 5k and at that point went to 315's. I dropped down to a true 20.3 before correcting for the tire change with Jscan. After correcting on the first couple of tanks I have an average of 21.9 mpg in combined driving. I will drive several thousand miles after the lift and report back. I only post from my personal experience and not what I read or had seen somewhere. I simply reported my experience to tell others what is possible and not what they will experience. I had a Charger Hellcat and averaged over 21 mpg. I had a Ram 2500 with the 6.4 and averaged about the same. It's all in how you drive.

Every brand new vehicle gets it's worst fuel economy before the 1st few oil changes so you can't really compare right after trade in. There's 0 chance with the 8 speed auto that you are seeing that 400rpm drop anywhere except downhill with a tail wind. It will simply drive around typically 1 gear lower than it would have when you go from 33s to 37s. It's simple physics, there's more wind resistance and rolling resistance with a larger tire, especially when you are comparing a tire a full 4" larger. It takes more energy to move the same object in the exact same conditions with more wind resistance period. The problem is "exact same conditions" is impossible. Even on the same route the wind could be far different that day. As you've said, it's all in how you drive. Accelerating and breaking even slightly less aggressively will make a huge difference. Bottom line if you're getting better fuel economy on bigger tires you aren't in the exact same conditions. On the same cross country roadtrip in our lifted JLUR on 38s our wrost tank was 11.5mpg (lots of uphill, cruise control set at 90mph), our best was 22mpg (most flat, adaptive cruise set to 75mph, and drafting a big rig nearly the entire time at about 70mph). The difference is in the driving and bigger tires will never help fuel economy all things being equal.

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[Gladiatorx2]

In the example given by the OP he said that the weight of the OEM load range D tires was heavier than the load range C 35's that he put on his. So lease unsprung weight and less revolutions per mile as well as more inertia while going down hill off throttle. In his case given the right circumstances I could see a slight increase in economy at speeds under 65 and a marked decrease in city stop and go traffic. I will say again that driving style, speed, traffic, terrain,etc is very important. It literally takes me about a month to develop my driving to get maximum economy. I am not a scientist nor an engineer but simply a car guy that has experience in three models and years of multiple trims in our Gladiator 's. As far as the RPM range/difference it's not incremental on our tach's so it is not exact but by using simple math a 10% difference in diameter and tire revolutions per mile should equate to the same in 8th gear and would be effected by torque converter lock up and on or off throttle. I assure you that what I have been reporting her is my true experience. This is on a Rubicon, Sport S, and Sport S w/Max Tow. It also includes five different tire combinations five different wheels as well as my experience with the Max Tow both uncorrected as well as corrected. These are based on my experience and I would not expect others to get the same. Unless the drive 100 daily at average speeds just under 50 Mph in gently rolling terrain in East Tennessee with very light traffic

 

[DailyMoparGuy]

Okay…I’ll bite.

My MPG is up with 37s a few weeks after install. Computer now saying 14.9 mpg and was 15.3mpg prior to me drag racing with myself earlier today. It was high 13s and low 14s with 35s. This is pretty much all city/town and no changes to my driving style.

My speedometer matches GPS at 85 mph but idk if that’s the most accurate check. I’m going on a routine road trip to PA soon and will calculate the mileage myself. 35s gave me 18-19mpg usually.

Edit: My old 35s were E range. New 37s are D range. Maybe that’s part of it, but idk. Those old Kendas were very light compared to these Baja Bosses. Maybe I’m just a magician

 

[Chans4x4]

A follow up. Went a few tanks of gas and always got the same mileage but had a noticeable power loss as expected. Recently the computer must have learned and have the power back but consistent .5 MPG loss over 35s.

 

[Greg_L]

It is an illusion if you have told the computer you changed your tire size. Also, changing tire size doesn't actually change your differentials gear ratio. Your 4.10s are still 4.10s

It's not an illusion. Tire size absolutely affects effective final gear ratio,

 

[GI Grandpa]

It's not an illusion. Tire size absolutely affects effective final gear ratio,

How? If your Jeep came with 33s and 4.10s and you mount 37s but don't regear, you still have 4.10s in the differentials

 

[Greg_L]

How? If your Jeep came with 33s and 4.10s and you mount 37s but don't regear, you still have 4.10s in the differentials

I'm not gonna give the physics lesson about it. You can Google it if you're curious. Everyone from rock crawlers to drag racers get the concept. Tires and gears matter. Of course it doesn't literally change your physical gearing. Yes, you'll still have 4.10s. But the larger tire makes them seem numerically lower to the engine. Add in the crazy overdrive ratio of the 8th gear in the AT and you're dipping into low low RPMs at speed. So low that sometimes the vehicle's onboard computerized bullshit won't allow the shift into 8th. That's the conundrum that people seem to be struggling with. And...the potential lack of low end torque.

Play with this little tool.
https://tiresize.com/gear-ratio-calculator/

Now the real question is....does it even matter? It does for some people. It depends on how you use or want to use your Gladiator. I run 37s with 3.73 gears and I'm quite happy with how the truck behaves. I don't live anywhere near or do any hardcore off-roading. I'm a city slicker in a very flat coastal city 99% of the time. I mostly hit beaches and swampy shit in the marshes. So my effective 3.42-ish gear ratio is ok with me. It's nice around town and still plenty adequate in the muck.

 

[GI Grandpa]

I'm not gonna give the physics lesson about it. You can Google it if you're curious. Everyone from rock crawlers to drag racers get the concept. Tires and gears matter. Of course it doesn't literally change your physical gearing. Yes, you'll still have 4.10s. But the larger tire makes them seem numerically lower to the engine. Add in the crazy overdrive ratio of the 8th gear in the AT and you're dipping into low low RPMs at speed. So low that sometimes the vehicle's onboard computerized bullshit won't allow the shift into 8th. That's the conundrum that people seem to be struggling with. And...the potential lack of low end torque.

Play with this little tool.
https://tiresize.com/gear-ratio-calculator/

Now the real question is....does it even matter? It does for some people. It depends on how you use or want to use your Gladiator. I run 37s with 3.73 gears and I'm quite happy with how the truck behaves. I don't live anywhere near or do any hardcore off-roading. I'm a city slicker in a very flat coastal city 99% of the time. I mostly hit beaches and swampy shit in the marshes. So my effective 3.42-ish gear ratio is ok with me. It's nice around town and still plenty adequate in the muck.

I think you are talking final drive ratio, and I agree, that does change if you change tire size, regear, shift gears or change your transfer case.

 

[Greg_L]

I think you are talking final drive ratio, and I agree, that does change if you change tire size, regear, shift gears or change your transfer case.

It changes across the board. It affects the entire spectrum. Your transmission and transfer case can't pick and choose when and where the tire size/gear ratio goes from actual to effective.

 

[HCoopo]

My mpg also improved by 1mpg going from a stock Mojave to AEV 2" and 37s.

I went from 15 to 16 mpg. Yes they are KO2s which are very light and I chose 17" wheels which are also very light. I reprogrammed the comp to say I have 35.75" tires. I had the Falken MTs and they were horribly loud and probably had more rolling resistance compared to AT KO2s. I"m just happy the MPG didn't plummet.

What wheels are these?? They look great.

 

[Zachanadandy]

Also I have hand figured my fuel economy on all three Gladiator's as well as tracked it on the EVIC which is pretty close to accurate. With the larger tires and no other changes your RPM is lower at any given speed. On my Rubicon w/37's I drove it nearly 500 miles to trade it in on this Gladiator with Max Tow. This was through East Tennessee up to Norfolk, Virginia on far from flat ground. I got 23 MPG hand figured. On the Max Tow coming back OEM 245/75r17's I struggled to even hit 21 MPG. Same route, same gear, same driver. The change was in RPM at say 65 was 350 to 400 RPM higher depending upon torque converter lock up.

The torque converter is locked up 100% of the time above 3rd gear...

 

[Sigz]

Okay I've decided on either 37's BFG Ko2 35x12.5x17 or 35's Toyo Open Country AT3 35x12.5x17 with 3.73.

This thread gives me some hope that the lighter weight BFGs would still be okay with the 3.73.

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