96R50

Tire circumference is Pi x D. A 31” tire is 31” x 3.14 = 97.34”.

12” = 1’ so 97.34”/12” = 8.11’. For every revolution the car moves 8.11’.

5,280’ = 1 mile so 5,280’/8.11’ = 651 revolutions per mile.

For your stockers, 235 is the section width in mm. 75 is the percentage of the section width that is the side wall and 15 is the rim diameter in inches.

235mm x (75% x 2) = 235mm x 150% = 352.5 mm Remember that the diameter is side wall-rim-side wall. That is why the 75% is multiplied by 2.

1mm = 0.03937” so 352.55 x 0.03937” = 13.88” + 15” (rim) = 28.88” for the stock tire diameter. Following the formulas for the 31” tire the calculation is:

28.88” x 3.14 = 90.68”/12” = 7.56’ per revolution. 5,280’/7.56’ = 698.4 revolutions per mile.

698.4 (stock) / 651 (31” tire) = 1.07

For every mile clocked on the odometer you actually drove 1.07 miles. If you change your oil every 3,000 miles by the odometer the actual mileage is 3,000 miles x 1.07 = 3,210 miles.

Keep in mind that a 31” tire doesn’t actually measure 31” and I suspect that a 235/75-15 isn’t really 28.88”.