runnin 31s

i wonder if some of you guys could help me out. im thinking of running 31s on my 5.2l zj but with the cost of gas and gas mileage always and issue now a days i was wondering if you could tell me how much running 31s has effected the gas mileage, with the added roling mass and all. i want bigger meats on my jeep but im not really fond of getting worse mileage on a jeep that isnt getting great mileage in the first place.
thanks,

I think you've answered your own question, mr_jones. You'll get worse mileage. Unless you do a lot of offroading and need the 31's you might be better off keeping what you have.

Here's a couple of sites that will compare wheel/tire sizes. Although they don't speak about mpg, they do show the % difference in size of tire, and you might be able to do a rough estimate.

http://www.dakota-truck.net/TIRECALC/tirecalc.html

http://www.1010tires.com/TireSizeCalculator.asp (http://www.1010tires.com/TireSizeCalculator.asp)

It's a little worse. But if you can keep off the gas it's not too noticeable. Have you added any lift? If not, then you may get some rubbing with 31s.
ByronP