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

[Jeeperjamie]

Air temperature, speed, terrain, all make big differences. The driver's foot is also one very big part of the equation.
My wife and I have both noticed drops in mpg as the weather has cooled. And on every trip we've made, including today going to Overland Park KS and back, speed was a huge factor. 73 mph sucked mpg-wise, but going down to 65 I could see a difference. Then when we got into traffic near Des Moines with even lower limits- mpg climbed even more.

Same here. I've noticed it's become more difficult for me to maintain 17+ Mpgs in mine as well. This will be the 1st full winter for me on 37's and since the weather has gotten into the 20's the last few weeks in the mornings, my Mpgs have dropped just below 17, I'm around 16.7 now. I can get it back up above 17 on the warmer days like today and tomorrow, I'll probably end up back at 17 before I get home tomorrow afternoon. Definitely noticing the same pattern above 70 mph as well.

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[Tim]

Same here. I've noticed it's become more difficult for me to maintain 17+ Mpgs in mine as well. This will be the 1st full winter for me on 37's and since the weather has gotten into the 20's the last few weeks in the mornings, my Mpgs have dropped just below 17, I'm around 16.7 now. I can get it back up above 17 on the warmer days like today and tomorrow, I'll probably end up back at 17 before I get home tomorrow afternoon. Definitely noticing the same pattern above 70 mph as well.

Personally, I wasn’t hung up on the lack of fuel efficiency of my lifted Jeep on big tires but reading that actually makes me feel a little better. Don’t take it the wrong way, I am not glad to see your MPGs drop, but rather I am glad there is some consistency amongst the trucks. It’s wild to read some of the posts claiming 20+ MPGs with lifted trucks on 37s and the 3.6. Nuances of specific builds aside, seems like 15 MPG for a heavier wheel and tire combo, stock gears, steel bumpers, winches, bed racks, etc. is not unreasonable whereas a lighter build and appropriate gears can take that up to 17 or so. Warmer climate can help things even more. That all makes perfect sense.

 

[bleda2002]

Also keep in mind where you normally drive and if you use the remote start makes a huge difference. Around my house every road is 55 but with a stop light literally every 2-3 miles, I average 13.5 driving around here because of the inefficiency of going 0-60-0 repeatedly. If I branch out and do longer drives that include more highway and more consistent driving I can get about 16 mpg with out remote start. Remote start for me is basically a 1mpg killer.

 

[Jeeperjamie]

Personally, I wasn’t hung up on the lack of fuel efficiency of my lifted Jeep on big tires but reading that actually makes me feel a little better. Don’t take it the wrong way, I am not glad to see your MPGs drop, but rather I am glad there is some consistency amongst the trucks. It’s wild to read some of the posts claiming 20+ MPGs with lifted trucks on 37s and the 3.6. Nuances of specific builds aside, seems like 15 MPG for a heavier wheel and tire combo, stock gears, steel bumpers, winches, bed racks, etc. is not unreasonable whereas a lighter build and appropriate gears can take that up to 17 or so. Warmer climate can help things even more. That all makes perfect sense.

Honestly I was expecting it to drop some. When I was running the 315 70 17 size tires before the 37's when winter hit the Mpgs dropped from 18.7 down to around 18.2 so I knew it would be tough for me to keep it above 17 the colder it got. Good thing about North Carolina is it may be 30 and snowing one day and the next day be close to 70.

As far as getting 20+ on 37's, unless you have a diesel then I'm calling BS on it all day with stock gearing. I don't care if you have everything stock and are running 37's you aren't going to ever see 20mpg unless you just reset the computer and your coasting downhill. I'm not trying to call people liars, but I will throw out the ole BS flag on it.

 

[Deleted member 30519]

Mine got down to like 14mpg but I think it's because I idle the jeep a lot. I usually sit in the JT before I go into work and eat my lunch listening to XM radio so sometimes its idling 20-25 minutes. When in reset the mpg I get 18 mpg easy after a day of driving a round trip commute.

 

[Deleted member 30519]

Mine got down to like 14mpg but I think it's because I idle the jeep a lot. I usually sit in the JT before I go into work and eat my lunch listening to XM radio so sometimes 20-25 minutes of idle. When in reset the mpg I get a 18 mpg easy after a day of driving a round trip. 38s and 5.13 gears.

 

[bleda2002]

Honestly I was expecting it to drop some. When I was running the 315 70 17 size tires before the 37's when winter hit the Mpgs dropped from 18.7 down to around 18.2 so I knew it would be tough for me to keep it above 17 the colder it got. Good thing about North Carolina is it may be 30 and snowing one day and the next day be close to 70.

As far as getting 20+ on 37's, unless you have a diesel then I'm calling BS on it all day with stock gearing. I don't care if you have everything stock and are running 37's you aren't going to ever see 20mpg unless you just reset the computer and your coasting downhill. I'm not trying to call people liars, but I will throw out the ole BS flag on it.

I'm on 37s, 55 gets me between 20 and 22 depending on how flat. But no way I'd keep that over a tank unless I accelerated really slow and them drove 55 the whole tank with out any other start stops or high speed passing or going over 55.

 

[Jeeperjamie]

Mine got down to like 14mpg but I think it's because I idle the jeep a lot. I usually sit in the JT before I go into work and eat my lunch listening to XM radio so sometimes its idling 20-25 minutes. When in reset the mpg I get 18 mpg easy after a day of driving a round trip commute.

Yeah Idle will kill the Mpgs for sure. These thing suck gas for some reason when sitting idle, that's why I let the ESS start/stop do it's thing when I'm in heavy traffic because if you don't the Mpgs will suffer.

 

[Jeeperjamie]

I'm on 37s, 55 gets me between 20 and 22 depending on how flat. But no way I'd keep that over a tank unless I accelerated really slow and them drove 55 the whole tank with out any other start stops or high speed passing or going over 55.

I mean on flat land at 55 I can see 30 Mpgs sometimes, but there ain't no way I could hold that. Be nice if everything was flat and we could all stay under 60. In this video on flat land at 65 I'm getting between 23-30 but to hold that for a average total would be impossible.

 

[MMMII]

I’ll add a data point.

-Rubicon
-35’s
-mopar lift
-Rebel Off-road half-rack
-steel bumper

I’m getting about 15.1. Thinking about taking off the wings of the bumper for a while to run it as a stubby just to see how that impacts my mpg’s.

 

[Jeeperjamie]

I’ll add a data point.

-Rubicon
-35’s
-mopar lift
-Rebel Off-road half-rack
-steel bumper

I’m getting about 15.1. Thinking about taking off the wings of the bumper for a while to run it as a stubby just to see how that impacts my mpg’s.

When you do that I'd be willing to bet it drops even more. More air flow up under the fenders is going to cause more wind drag and less fuel economy, how much less is anybody's guess.

 

[ShadowsPapa]

....................

As far as getting 20+ on 37's, unless you have a diesel then I'm calling BS on it all day with stock gearing. I don't care if you have everything stock and are running 37's you aren't going to ever see 20mpg unless you just reset the computer and your coasting downhill. I'm not trying to call people liars, but I will throw out the ole BS flag on it.

I have a feeling some of them are simply plugging in the numbers they measure of the tire and calling it accurate. It's not.
They need to verify those numbers using the odometer. If I went by what I measure, my numbers would be skewed, too. I had to adjust the numbers twice and finally verified with the odometer.
But to simply measure or put in what someone told them to - no.
Verify with a GPS against the truck's odometer.

 

[ShadowsPapa]

When you do that I'd be willing to bet it drops even more. More air flow up under the fenders is going to cause more wind drag and less fuel economy, how much less is anybody's guess.

I wonder how many have paid close attention to the modern design of semi tractors. The cab these days makes sure the fronts and sides of the tires are not exposed to the wind of travel any more than necessary. Some of those cabs cover 4/5 of the front of the tire. I've passed a couple that actually had flat plates on the wheels to cover the wheel dish on the rear wheels and on the trailer wheels. Those deep dishes catch wind. And we've all for many years seen those ground effects that direct wind away from the rear wheels on the trailers.
Even the dish of our wheels matters - so a deeper dish wheel is more drag. Frontage of the tire exposed is drag and the more aggressive the tread, the more drag.
The tire's foot print being larger - front to back (diameter) and side to side (width of the tire) matters as does tread design.
Lift it and more air gets tangled up in the fender wells and under the vehicle. Modern vehicles looking for MPG have a smooth undercarriage. When I have to clean bugs off my suspension and steering - I know I'm catching a lot of wind with the suspension and steering.
The normal wind speed on a day with any wind is higher off the ground than at ground level - so you raise the truck into higher wind speeds as well as catching air and causing drag under the vehicle.

 

[syreeves]

I use the Fuel Up app. Over 11,063 miles i have averaged 14.8mpg. That includes a fair bit (80%) highway driving, some light towing (boy scout troop trailer build shaped like a sail) and a couple of long road trips from Minnesota to Kansas, South Dakota and Washington DC. These numbers come from fuel pump gallons pumped calculated against odometer miles.

I run 35s on a JT Rubicon 6MT, stock gears, and a 2inch mopar lift. No tent or anything but my Ace Upper Decker adds about 200Lbs. I've had the rock rails and bumper on for about 10% of those miles (just got busy you know!).

 

[Jeeperjamie]

I wonder how many have paid close attention to the modern design of semi tractors. The cab these days makes sure the fronts and sides of the tires are not exposed to the wind of travel any more than necessary. Some of those cabs cover 4/5 of the front of the tire. I've passed a couple that actually had flat plates on the wheels to cover the wheel dish on the rear wheels and on the trailer wheels. Those deep dishes catch wind. And we've all for many years seen those ground effects that direct wind away from the rear wheels on the trailers.
Even the dish of our wheels matters - so a deeper dish wheel is more drag. Frontage of the tire exposed is drag and the more aggressive the tread, the more drag.
The tire's foot print being larger - front to back (diameter) and side to side (width of the tire) matters as does tread design.
Lift it and more air gets tangled up in the fender wells and under the vehicle. Modern vehicles looking for MPG have a smooth undercarriage. When I have to clean bugs off my suspension and steering - I know I'm catching a lot of wind with the suspension and steering.
The normal wind speed on a day with any wind is higher off the ground than at ground level - so you raise the truck into higher wind speeds as well as catching air and causing drag under the vehicle.

Yep, 100% agree. Same reason I stated before that I went full width on the bumper and went with a less aggressive treqd design this time over a Mud terrain. I know it's not going to get great gas mileage but there are things you can do to control how bad or good it is just by thinking out your build. People will
Argue with me all day that a light bar above the windshield will not affect Mpgs but anything you put up there that blocks air from flowing freely around the vehicle is going to cause drag, and then will cause Mpgs to go down.

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