General Question about size of tires and gas mileage?

[Xjforme]

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.

[jmsull]

I have a '99 4.0L auto/242 and 3.55. I have 235/85/16 which are 31.7", I get 12 mpg.

[XJ_ranger]

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

[BrettM]

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.

[bigwhitey]

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.

[bubba]

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.