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General Question about size of tires and gas mileage?

Xjforme

NAXJA Forum User
Can anyone tell me the difference from upgrading tire size and the difference in gas mileage with stock 3:55 gears? I'm currently running 30x9.5x15 and would like to upgrade to 31 or 32 depending on what type of mileage I will get. I'm currently getting 15 mpg on average from a 00 xj 4.0L and a Auto.
 
I have a '99 4.0L auto/242 and 3.55. I have 235/85/16 which are 31.7", I get 12 mpg.
 
I get 15-16 mpg on 33's

4.0 / ax-15 / 231 / ford 9" / 3.50 gears

for what its worth - 1 year ago i got 18 mpg with

4.0 / ax-15 / 231 / Dana35 / 3.07's on 235/75's
 
you will definitely lose a few MPG both because of the added rotating mass of the larger tires and the worse gearing. If you were to regear to 4.10 or 4.56 there is a good chance you would get better gas mileage than you do now. I got 20mpg at 65mph with 33x12.50s and 4.88 gears.
 
Going from 3.55's and 31's to 33's degraded my intown driving dramaticly. However with 4.88 gears I think I'm on the opposite end because I still get 15MPG like with the 31's. I bet I'll get awsome milage with 35"s.
 
I just installed my lift in my 98. I do mostly highway driving. I have the 4.0 aw4, 242, 31's and 3.55 gears. On my fill up today I got 19.2 mpg. And that was with towing my camper about 150 miles and doing some wheeling. But I also have one of those fancy K&N airfilters. MY mileage sucked to begin with but I aired my tires up to 40 PSI. It rides a little rough but I will suffer with a rough ride to get 19 mpg.
 
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