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TIRES VS. MPG.

C

CMAHON

NAXJA Forum User
Location
Arlington, TX
i just installed a 6in lift w/ 35's (stock gearing 3.55 for now) my mpg went to 10mpg if i'm lucky, is this normal. it was 12-13 highway before.
 
Remember your odometer is off, I used my GPS to calculate, I have the same set up and get 16 mpg on the highway. Higher than 70 mph and it drops and hills kill the gas mileage
 
Dam i have 33's with 488's, i only get 15 hwy..Thank god i did a tune up on her..It's a jeep that you lifted an put big tires on it..You want better MPG, go back to stock..
 
When you upgraded to 35's with your stock 3.55 gearing, did you change your speedometer gear? If not, you need to adjust your tank mileage accordingly to compensate (as your odometer will read less mileage than what was actually traveled).

So, assuming you're on stock gearing with a stock speedometer gear, you'll want to multiply your calculated mileage (or the distance as tracked by your odometer) by ~1.24 to get the "actual" mileage. This ratio was determined by simply dividing your new overall tire diameter (35") by the stock 225/75R15 tire diameter (~28.3"). Not perfect, but a good estimate.
 
CMAHON said:
i just installed a 6in lift w/ 35's (stock gearing 3.55 for now) my mpg went to 10mpg if i'm lucky, is this normal. it was 12-13 highway before.
Click to expand...

As others have posted.... 12-13 mpg with a stock Jeep is rather low...
The speedometer calibration is way off after going from tires 27" to 34" tires.
You have created an overdrive affect by lowering the affective RPMs that your engine is running at which affects acceleration.
The gas mileage is also affected since your foot is into the gas pedal alot more trying to accelerate....
To accomodate the 35" tires; you will need to upgrade the gearing to a minimum of 4.56, but 4.88s would be better...
Wait until after you have upgraded your gearing before purchasing a speedometer gear to calibrate your speedometer....
I have a 4.0L 5 speed with 4.56 gears and 32x11.50s and get about 15-16 mpg HWY when I keep the speed down to 65 mph with the A/C off.
 
damn who pissed in your cereal Scott. dont like the question then read a different thread. i never onced complained about it... it was a question. anyway what is so wrong about trying to find a way to go big and keep mpg up!!!
 
the way to fix it is gearing. I'm comfortable with the crappy MPG I get, but I don't drive it too much. Even with a gps I get maybe 14 on the highway and 8-10 in the city. BUT I have a TPS issue and stock gears. it is what it is.
 
thanks Anthrax i forgot to compensate the speedo being off. with that (1.24 thing) it put me on par with were i should be.
charles i want to stay stock 8.25 so 4.56 i think i my biggest option and i will wait to change the spedo gears

thanks for the help all!!
 
I was told that the carrier wasn't big enought for 4;88's but if there available then that is what i want. who is this sponsor. i will also search
 
It seems to bog the autos down more than the manuals. I am on 35s and 3.50 (ford ratio :) ) with a 5 speed and get 17-21 highway. It sure hates the highway hills, though. About 13-14 city.

4.56 or 4.88's are a good swap, although on the highway, it is not a guaranteed solution. Gear downs help more in the city stop-n-go than highway. When it comes down to it, it is revving higher to go the same speed, and that takes more gas. Whether that increase in consumption is overrun or not by the lessened bog factor on the hills is very debatable, though. With my 5 speed, I went to Moab in 4th gear once, to see how it did at about 2500 RPM instead of the 1800 RPM 5th gear does at the same speed (65) and it went down to about 15MPG (from a 18-21 Moab average)

Food for thought. Despite my decent MPG, I'd still rather be lower...
 
IIRC it was the D30 that was the limiting factor when it came to gearing lower than 4.56, but they are now selling 4.88s. I know Scott's running them in his, and I will be in mine soon.
 
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