Speedometer Correction Calculator

A taller or shorter tire changes how far the vehicle actually travels per wheel rotation — but the speedometer keeps counting rotations as if nothing changed. Enter your factory tire size and your new one to see the real gap.

Factory (old) tire

New tire

This is exact tire-diameter geometry, not a claim about your specific vehicle's calibration. A handful of newer trucks and EVs recalibrate the speedometer automatically from GPS or a TPMS-linked system — if yours does, this error may already be corrected. When in doubt, check against a GPS speed app at a steady speed.

Why this happens

The speedometer doesn't measure speed directly — it counts wheel (or output-shaft) rotations per second and multiplies by the factory tire's rolling circumference to get a speed. Fit a taller tire and each rotation covers more ground than the calibration assumes, so at any given rotation rate the true speed is higher than what's shown. A shorter tire does the reverse: true speed is lower than shown.

The error is proportional to the diameter change and does not depend on speed itself — a 4% taller tire means the speedometer under-reads by 4% at 30 mph and by 4% at 80 mph alike. That's why the table below is a single flat percentage, not a curve.

Odometer and trip mileage

The odometer runs off the same rotation count, so it drifts by the identical percentage: a taller tire means you're driving further than the odometer shows, which quietly pushes out real-world fuel-economy numbers and mileage-based maintenance intervals (oil changes, warranty mileage) later than the odometer implies. A shorter tire does the opposite — the odometer runs ahead of actual distance.

What to do about it

Under about 3% — most people leave it. It's inside the tolerance many factory speedometers already carry from the factory.
3–7% — worth knowing at a glance (add a few mph mentally at highway speed) or correcting with a tuner/programmer if your vehicle supports it.
Over 7% — get it recalibrated. Besides the speedometer itself, ABS and traction/stability control read wheel speed for their own logic, and a large enough mismatch between front and rear rolling diameter (staggered or mixed setups) can trigger false intervention, not just a wrong number on the dash.

Recalibration options depend on the vehicle: some infotainment systems have a built-in tire-size setting, aftermarket tuners/programmers can reflash the calibration constant, and on older vehicles a speedometer gear or a signal recalibrator in the speed-sensor line can do it mechanically.

Find a tire size that avoids this entirely →
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