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Stock tire = 29.5" - 92.63" circumference

New tire = 31.5" - 98.91" circumference

The CPU says I drove 226 miles since last fill up so here is how I came up with the number of revolutions the tire made during that time.

226miles

__x5280feet/mile__

1,193,280feet

1,193,280feet

__x 12inches/foot__

14,319,360inches

__14,319,360inches__

92.63inches = 154,586.6revs

That means that the computer thinks the tire made 154,586.6 revolutions since the last fill up. From there I think I can calculate the actual miles like this:

154,586.6revs

__x 98.91inches__<--new tire circumference

152,901,164.1inches

__152,901,164.1inches__

12inches/foot = 1,274,180.3feet

__1,274,180feet__

5280feet/mile = 241.3miles

This means that I actually went 241.3 miles on my last fill up even though the computer in the jeep says I went 226 miles. The computer also calculated the average MPG to 14.3. so.....

__241.3miles__

16.892gallons = 14.3MPG

If all of that is correct then how is it that the XK knows the correct MPG but not the correct mileage? or maybe it is just a strange coincidence.