Does tire size affect the odometer?
Answer
+3.49%
A speedometer calibrated for 265/70R17 and fitted with 285/70R17 reads low by 3.49%. An indicated 70 mph is an actual 72.4 mph.
The working
- Indicated 30 mph
- 31.0 mph actual
- Indicated 50 mph
- 51.7 mph actual
- Indicated 70 mph
- 72.4 mph actual
- Indicated 100 miles
- 103.5 miles actual
- Diameter change
- +1.10 in
The same error, in a form that accumulates
A speedometer that reads low is a momentary inconvenience. An odometer driven by the same wheel rotations is out by the identical percentage, and it never catches up. Both instruments count the same thing, so the two errors are not merely similar — they are one error displayed in two units.
Fitting 285/70R17 where 265/70R17 was calibrated gives 3.49%. For every 100 miles the display counts, 103.5 have passed under the wheels.
What that looks like over a year
At around 10,000 displayed miles, roughly 350 miles go unrecorded. Service intervals arrive later than the display suggests by the same proportion, warranty and lease mileage are undercounted, and the figure that eventually appears on a sale listing understates the real total. None of that is dramatic at 3.49%, and all of it is permanent for as long as the tires are fitted.
The fuel-economy readout inherits the same error, because it divides a distance the vehicle believes by a fuel volume it measures directly. Under-counted miles produce an under-stated figure, so the displayed economy drops by roughly the same 3.49% even though nothing about consumption has changed.
Direction matters and is easy to get backwards
A taller tire makes both instruments read low: fewer rotations per mile, so fewer counted miles. A shorter tire does the reverse and inflates the recorded total. Because most changes people make are upward, the usual outcome is a vehicle that has covered more ground than it admits to.
Recalibration reaches one and not always the other
Where a vehicle allows a tire-size entry, the correction is applied to the rotation-to-distance conversion and both readouts follow. Where the fix is an aftermarket module or a gear change in a mechanical drive, it is worth confirming that distance is being corrected and not only speed. Run the pair through the calculator to see the distance column beside the speed one, or read how a speedometer is calibrated for where in the system the number is produced.