I'll be needing to replace the tires on my Jeep in the near future. I'd really like to go with a taller/bigger tire, but I'm concerned about how it will effect gas mileage. I'm already not too impressed with the mileage I'm getting and making it worse certainly isn't a good idea. My normal yackin' around mileage is 16.something. I've hit 19.7 on a trip, but right now, on winter blend, with lots of morning idling I'm down to 15 and sometimes in the 14s. I really thought the XJ Cherokees would do better than most Mopar 4x4s of similar vintage.

Anyway, my current tire size is 235 65 R17. I've been unable to find specs on this size. I was using the tire section on Jegs site to get specs on 245s and other sizes. I'd just like to know what size I could jump up to without having an effect on mpgs. I also have to consider clearance issues. (I have a 3 inch lift.)

So based on what I already have, what do you think a good size would be for replacement tires?

Also, does a more aggressive tread design (think off road) vs. a regular street design have a negative impact on mileage? Any experiences?