Calculating actual mileage/MPG with oversized tires and stock gears
Make sure my math is correct.
Say your odometer reads 287mi and you just put 16 gal of fuel in your tank, your mileage would be 17.93mpg BUT you have 31" tires (or larger) Stock tires on an XJ are 235/75R15 So doing the math to get that in a standard size would be Code:
235mm X .75 = 176.25mm/25.4 (mm in an inch)= 6.93" We have to find by what percentage the new tires are larger. 31"28.86"= 2.14"/.2886 (this is one percent of the original tire size) =7.4% So actual mileage would be 287mi + 7.4% = 308.2mi Code:
308.2/16= 19.3mpg 28.86 X 3.14 (pie.....:drool: )= 90.62 31 X 3.14 (p...i...e..... :drool::drool: )= 97.34 97.34  90.62 = 6.72/.9062 = 7.4% < amazing isn't it.....ratios are the same because the circumference of a circle depends entirely on its diameter times a tasty pastry containing a filling of sweet ingredients. So if you had say 33" tires on stock gears.... Code:
33"28.86" = 4.14"/.2886 = 14.34% 
That's awesome, except you're using listed diameter and not real world measurements, so all your stuff is off.
Best thing to do by far is use GPS to verify the true difference in your speed vs. what the speedometer shows, then use that percentage to add to what your odometer is reading when you fill the tank. 
The math does seem unnecessarily complicated and likely to show you an inaccurate number. I used mile markers on the freeway on a long drive.
e.g. Went from 108 to 208, but my odometer showed a difference of 110. 
I agree that real world measurements would be different, but those measurements could be found fairly easily with a real world measuring device.(in that case I agree that circumference would be a much more accurate)
I still think its close, and defiantly over complicated haha! Guess I was bored and my mind got to wondering.......happens way to often Sent from my phone where correct grammar is difficult. 
31 inch tires on stock gears for a 96 zj, what will the difference in miles be on the odometer? my stock were 225/70/16 @ 710.5 revs per mile. The new tires are 31/10.50/15 @ 685 revs per mile. So if I travel 200 miles what will the odometer read for stock vs larger tires? If stock was 200 miles, then the larger tires will read less miles on the odometer, right?

So I think I figured it out. My odom reads .8 for every actual mile I travel. So 200 miles on the odom is actually 250 miles traveled. So my last tank I figured 11.11 mpg. But since it is .2 off, it was actually 13.8 mpg. All city! Makes me feel better especially since I am lifted and have 31 inch tires.

225/70/16 calculates to 28.4"
3128.4 = 2.6/.284 = 9.15% So take your odometer reading plus 9.15% and that's your actual miles traveled. 200 odometer miles = 218.3 actual miles Sent from my phone where correct grammar is difficult. 
See, every tire runs different. One 31x10.50 will be slightly different than the next 31x10.50. Also, the markers on the highway aren't perfect. They are installed by workers who measure off a mile at a time and there's lots of room for error.
I'm telling you guys... the easy way to do this and eliminate error is to use GPS and see what the % difference is between true and speedometer. Then add or subtract that % as needed to the odometer reading when you fill up the tank. If you do it this way, it takes all the measurement and guesswork and variances out of the equation. 

I just round it off to 10%. 200 miles on 31's would be about 220+. The XJ was also calibrated to a 225/75R15, not a 235.

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Also didn't realize 225/75/15s were actual stock size. Sent from my phone where correct grammar is difficult. 
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The speedo gear is SO easy to remove and change. Why bother doing all the math when you could just pop a proper gear in there to make you odometer and speedometer accurate again? Sent from my phone 
my stock tire size per my sticker on the door was 225/75/16

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