hrm... well, its going to be kind of hard. Your speedo gear is wrong, so your millage is wrong as well. You can't trust your odometer. I've never heard of trying to do the math to figure out the difference, but it would involve firguring out the % difference in the circumference of the tire and multiplying it times the millage you have.
So, if you got 200 Miles on your jeep, and your tire is 3% larger. You would have to multiply 200 * 1.03 and devide that by gallons used.
Are you tires ACTUALLY 31"? or is that just what they read on the side? For example, my tires are 33s but if you measure them, they are actually about 32.5"
Are stock tires 29"? I forget.