13.1mpg best I can get with 35’s. Questions inside…..

[Aonarch]

If you are concerned with MPG, go with 4.88s with the 37s.

As mentioned wheel and tire weight drastically effect MPG. Those are heavy wheels and tires.

If you aren't attached to those wheels, you might want to throw them up for sale and get something lighter. Tirerack says those are 35.3 lbs each. My wheels are 25 lbs each. Your tires are also 10 lbs heavier than my 35s.

A rough equation for calculating unsprung weight is to multiply it by 4. So four wheels and tires multiplied by 4. Do the same with the stock weight and that is the weight impact to your MPG.

Same goes for the tires, pay attention to weight and load rating while shopping.

Don't rely on the trip computer, manually compute your MPG next time.

Let's says the stock wheels and tires are 70 lbs combined, that is 1120 of unsprung weight being pushed down the road. Your combo is something like 1728 lbs. Then combine the larger footprint for the wind to move.

I'd think the stubby bumper would effect MPGs are well, there is a reason why the base Sport has the plastic bumper to fender connector.

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[WAOverlandJT]

Dunno what you've been hearing - but no, no issues. And if you don't have it in 4H auto, it has no impact on anything, it's like any other t-case in 2 wheel mode.
Heavy tires and heavy wheels matter - I know what others will say "I have 35s and am fine" - I have to laugh a bit because a 35 can vary almost a full inch in diameter and many pounds in weight. So unless someone says they have the exact same brand and size tires in the same load range - you can't really compare.
These are also very picky on weight and the frontage presented to wind.
I noticed a difference when swapping bumpers. If air hits the fronts of the front tires, it's more wind resistance. The plastic bumpers prevent that, and the sport that has the extra piece between bumper and front edge of fender prevents that even more.
I used to get 23 and more very easily. Now I'm about 20-21 most of the time.
I changed to A/T tires a tad wider, added a steel bumper, etc. - and lost a good 2 to 3 mpg with my changes.

Yeah definitely can be a big difference with weight at the wheel. These are really like 34.3” and 65 lbs since it’s a 315/70/17.

Eventually they will get a tuner for the 21 and I can play around with that. Hopefully the gears help when I eventually go to a 37 or I will be at like 11 mpg around town with mixed driving I guess. Really wish I would have just ordered a diesel.

 

[Aonarch]

Yeah definitely can be a big difference with weight at the wheel. These are really like 34.3” and 65 lbs since it’s a 315/70/17.

Eventually they will get a tuner for the 21 and I can play around with that. Hopefully the gears help when I eventually go to a 37 or I will be at like 11 mpg around town with mixed driving I guess. Really wish I would have just ordered a diesel.

$4000 plus added maintenance expenses buys a lot of gasoline.

 

[ShadowsPapa]

I find it ironic that people want huge heavy tires and wheels - then even think about mpg.
Sorry, but they are pretty much contradictory. There's a reason Jeep puts on the tires they do - CAFE and MPG - any variation from that will impact MPG negatively. 35 are bad mpg and then going to 37s?

Unsprung weight is bad for suspension and steering and heavier tires and wheels impact braking in a negative way - and for sure mpg. It's really pretty simple.

As far as mpg calculations - make sure to use an independent GPS to verify tire size input.
Don't go by measurements or what others may say to put in.
Plug in the numbers - and then verify with a GPS!
Very possible it's still wrong if you haven't actually driven a few miles and compared odometer to GPS readings.

 

[WAOverlandJT]

If you are concerned with MPG, go with 4.88s with the 37s.

As mentioned wheel and tire weight drastically effect MPG. Those are heavy wheels and tires.

If you aren't attached to those wheels, you might want to throw them up for sale and get something lighter. Tirerack says those are 35.3 lbs each. My wheels are 25 lbs each. Your tires are also 10 lbs heavier than my 35s.

A rough equation for calculating unsprung weight is to multiply it by 4. So four wheels and tires multiplied by 4. Do the same with the stock weight and that is the weight impact to your MPG.

Same goes for the tires, pay attention to weight and load rating while shopping.

Don't rely on the trip computer, manually compute your MPG next time.

Let's says the stock wheels and tires are 70 lbs combined, that is 1120 of unsprung weight being pushed down the road. Your combo is something like 1728 lbs. Then combine the larger footprint for the wind to move.

I'd think the stubby bumper would effect MPGs are well, there is a reason why the base Sport has the plastic bumper to fender connector.

If you are concerned with MPG, go with 4.88s with the 37s.

As mentioned wheel and tire weight drastically effect MPG. Those are heavy wheels and tires.

If you aren't attached to those wheels, you might want to throw them up for sale and get something lighter. Tirerack says those are 35.3 lbs each. My wheels are 25 lbs each. Your tires are also 10 lbs heavier than my 35s.

A rough equation for calculating unsprung weight is to multiply it by 4. So four wheels and tires multiplied by 4. Do the same with the stock weight and that is the weight impact to your MPG.

Same goes for the tires, pay attention to weight and load rating while shopping.

Don't rely on the trip computer, manually compute your MPG next time.

Let's says the stock wheels and tires are 70 lbs combined, that is 1120 of unsprung weight being pushed down the road. Your combo is something like 1728 lbs. Then combine the larger footprint for the wind to move.

I'd think the stubby bumper would effect MPGs are well, there is a reason why the base Sport has the plastic bumper to fender connector.

Thanks, good tips and definitely some that I have been thinking about as I plan out this build in the future. I thought about doing 4.88’s before the 37’s but worried I would end up wishing I did 5.13’s. Never changed gears in any of my prior Jeeps yet. So always learning what I can.

 

[Aonarch]

Thanks, good tips and definitely some that I have been thinking about as I plan out this build in the future. I thought about doing 4.88’s before the 37’s but worried I would end up wishing I did 5.13’s. Never changed gears in any of my prior Jeeps yet. So always learning what I can.

4.88 is a good middle ground for 37s, 5.13s are great too, but obviously would raise your cruising RPM a little bit. If you were in Florida and were concerned with MPG, 4.56 are best.

But hell with elevation and hills, maybe 5.13 would be better for you. It could help prevent gear hunting, which kills MPG.

Does your Gladiator do a lot of gear hunting now?

I've posted a table with the actual gear ratios with changes to tire size, but hear is a simple formula that works with most vehicles.

Divide your new tire size by the old tire size. 35/33
then multiply that by your current gear ratio. Then round up to the closest aftermarket gear ratio.

37s on a Rubicon calls for 4.59, which is where my Florida comment comes from. 4.56 is great for both 35s and 37s, as long as they aren't heavy weights.

But for you, I'd go with 4.88s.

 

[WAOverlandJT]

I find it ironic that people want huge heavy tires and wheels - then even think about mpg.
Sorry, but they are pretty much contradictory. There's a reason Jeep puts on the tires they do - CAFE and MPG - any variation from that will impact MPG negatively. 35 are bad mpg and then going to 37s?

Unsprung weight is bad for suspension and steering and heavier tires and wheels impact braking in a negative way - and for sure mpg. It's really pretty simple.

As far as mpg calculations - make sure to use an independent GPS to verify tire size input.
Don't go by measurements or what others may say to put in.
Plug in the numbers - and then verify with a GPS!
Very possible it's still wrong if you haven't actually driven a few miles and compared odometer to GPS readings.

yeah it’s also a giant brick in the wind, so I don’t expect a miracle lol. These wheels and tires were on my turbo JL and then I started to get into overlanding with Covid. So now I’m much more future focused on weight where I can.

Still I rarely see people down at 13 mpg like I am in on discussion boards. Even with 37’s, wheels, bumpers. But could just be my combo and luck from the factory.

 

[WAOverlandJT]

4.88 is a good middle ground for 37s, 5.13s are great too, but obviously would raise your cruising RPM a little bit. If you were in Florida and were concerned with MPG, 4.56 are best.

But hell with elevation and hills, maybe 5.13 would be better for you. It could help prevent gear hunting, which kills MPG.

Does your Gladiator do a lot of gear hunting now?

Yeah I’m in the PNW, not nearly as flat lol. I constantly wonder wtf the Jeep is thinking with shifting. I can see 8th when It’s flat unless I get a good headwind or has any incline at all.

 

[Aonarch]

yeah it’s also a giant brick in the wind, so I don’t expect a miracle lol. These wheels and tires were on my turbo JL and then I started to get into overlanding with Covid. So now I’m much more future focused on weight where I can.

Still I rarely see people down at 13 mpg like I am in on discussion boards. Even with 37’s, wheels, bumpers. But could just be my combo and luck from the factory.

I'd say more driving style, location, and those heavy wheels and tires.

People are always quick to think something is wrong with the vehicle just because some stranger on a forum says they get miracle MPG

Source: been on car forums for 22 years.

 

[Aonarch]

Yeah I’m in the PNW, not nearly as flat lol. I constantly wonder wtf the Jeep is thinking with shifting. I can see 8th when It’s flat unless I get a good headwind or has any incline at all.

They designed the gearing, shift points, and ratios for the factory 33s. That's why.

 

[Walter-new jeeper]

It’s been just over 4k miles so far and mpg just seems to keep dropping. I even baby the S out this thing when driving it. I have a rack and roof top tent so I figured that’s killing it the most. I took it off about a week ago and still getting 13.1 with city/hwy mix. Everyone on this board is posting like 15-20 and many have more mods and bigger tires. So trying to figure out wtf I can do to get more range out of this thing.

I’m wondering if maybe the tazer could be causing an issue somehow. Going to take that off and test this week.

21 rubicon
35” Baja atz with 17” fuel zephyr wheels (kinda heavy wheels).
RR arcus stubby bumper
Taser mini- updated about 2 months ago to fix issues it had.
4h auto ( I wonder if this also drops mpg
I run regular fuel, so 87/89 normally.

Thoughts on stuff to try? I read the thread with the pulsar but that won’t work on the 21 I guess.

Don't have a RTT but do have a rack (fairly even with the roof) with a tonneu cover. Got a 2" lift, still on 33's. Around town mixed - usually 14.5-15 MPG. Here's what sucks - the cruise control. Did a long road trip from the Seattle area down to North Rim of the Grand Canyon, then to Moab, and then home. I used cruise control a lot on the highway due to my knees. Granted I was loaded with camping gear and stuff. On CC I averaged 14.5 highway. When I took it off CC for long stretches I could get it up to 16. On CC seldom got to 8th gear even on flat terrain doing 75-80. Off cruise control would spend plenty of time in 8th gear. I have the 3.6 Pentastar.

 

[Gren71]

It may just be worth your time And effort to go back to a stock tire size.

Not the answer you want I know.

Just food for thought.

I went through this process recently. Here is a link to the thread. It may have some details that help you figure out whats right for you.

https://www.jeepgladiatorforum.com/forum/threads/worth-it-to-go-back.47891/

 

[NachoRuby]

One more question I don't think was asked: what pressure are you running in your tires? I don't think I saw much of any hit going to 35s, but my 35s weigh the same as the factory 33s did.

 

[Tim]

I have 35” M/T Baja Boss AT’s on my Rubicon with 4.10s. Those are some heavy tires. My wheels are 28lbs each, I believe. I have a bed rack (no tent), steel bumper, and winch and I am getting 15mpg mixed.

As far as mpg calculations - make sure to use an independent GPS to verify tire size input.
Don't go by measurements or what others may say to put in.
Plug in the numbers - and then verify with a GPS!
Very possible it's still wrong if you haven't actually driven a few miles and compared odometer to GPS readings.

This is great advice and I will elaborate a bit. Don’t worry so much about the speedometer, but rather verify the odometer (as Shadowspapa suggests) against known distance. When I calibrated my speedometer to my GPS (used a Tazer to program tire size) my odometer was still off by quite a bit compared against mile marker posts on my commute home even though my speedometer was correct. I am lucky in that the highway I frequently travel is posted in .1 mile intervals. In any case, the speedo will read higher than the speed you are actually traveling when the tire size is correct. This is done intentionally from the factory as a safety margin and to accommodate future tire wear, etc. In any case, errors will cause the MPG as reported by the DIC to be incorrect.

 

[SnazzGlad]

I’m getting mid 13’s city driving only. Never going above 45 mph. Stock suspension sport s with the 32” Firestone MT2’s. Summer months it’s 15 but long highway trips I’m get 20+. Kind of apples to oranges with your setup but just throwing out my poor mileage too. I will be going to an A/T sometime in the near future.

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