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The number of turns your tire makes when travelling a mile directly influences the speedometer and odometer readings. The speedometer and odometer asume a specific number of turns in a mile. If your tires turn at some other speed, you will induce a measurement error. If the tire size is significantly different, the measurement error will be significant as well. The number of turns-per-mile is inversely proportional to the tire circumference. The circumference is based up tread width and aspect ratio as well as the rim diameter. If you make a calculation of circumference based only on these numbers, you will get the "unloaded" circumference. The "loaded circumference is less than that. The industry standard for estimating the loaded vs. unloaded difference is 3.1%. The loaded circumference is 3.1% less. Therefore there are about 3.1% more turns-per-mile than the uncorrected calculation predicts. The actual difference for a particular set of tires will depend on the inflation pressure, driving speed, treadwear, and flexibility of the particular tire sidewall. 3.1% is just an average number across many sizes and brands of tires.
Here are the formulas you can use for your turns-per-mile calculation.
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