Why new tires change your speedometer
Your speedometer does not measure speed directly. It counts how fast the transmission output or wheels turn and assumes the factory tire size and axle ratio. Taller tires travel further per turn, so the car goes faster than the needle shows and the odometer under-counts miles. A numerically higher axle ratio does the opposite, which is why re-gearing after bigger tires often brings the speedometer back close to correct.
Speedometer correction formula
From a GPS check: correction factor = GPS speed ÷ speedometer speed
Worked examples
Bigger truck tires: 265/70R17 to 285/75R17
The tires grow from 31.6 to 33.8 inches, a factor of 1.0696. When the speedometer shows 60 mph you are really doing 64.2 mph, and at 70 mph indicated you are at 74.9 mph. The odometer counts only about 935 miles for every 1,000 you drive.
Re-geared at the same time
Changing the axle from 3.73 to 4.10 with those tires gives 1.0696 × 3.73 ÷ 4.10 = 0.973, so the speedometer now reads about 2.7% high: 58.4 mph real at 60 indicated.
How to fix speedometer error
- Electronic recalibration: many trucks and SUVs can be reprogrammed with a handheld tuner or by a dealer to the new tire size.
- Gear change: older vehicles with a mechanical speedometer use a different driven gear in the transmission.
- Re-gear the axle after a large tire upgrade; it restores both speedometer accuracy and acceleration.
- Check with GPS on a straight, flat road at a steady speed, then use the GPS mode above.
Need tire diameters from the sidewall size? Use the tire size calculator. Planning a re-gear? Try the gear ratio calculator.
Frequently asked questions
How do I calculate speedometer error from tire size?
Divide the new tire diameter by the old one and multiply by the speed shown. Going from 31.6 to 33.8 inch tires, 60 mph on the speedometer is about 64.2 mph real speed.
Do bigger tires make you go faster than the speedometer says?
Yes. A taller tire covers more distance per turn, so your real speed is higher than indicated and the odometer records fewer miles than you actually drive.
Is it legal to drive with an inaccurate speedometer?
You are responsible for your real speed, and an inaccurate speedometer is not a defense against a speeding ticket. Recalibrate after changing tire size.
How accurate are factory speedometers?
Many factory speedometers read a little high on purpose, often by 1% to 3%, so the car is going slightly slower than shown. A GPS check reveals the exact difference.
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Last updated October 2026.