Key takeaways
- A larger new tire makes the speedometer read slower than your true speed.
- True speed = indicated × (new diameter ÷ old diameter).
- Error % = (new − old) ÷ old × 100 — stay within ±3% of stock.
- A bigger tire also makes the odometer under-count the miles you actually drive.
What speedometer error is
Speedometer error is the gap between the speed your dash displays and the speed you are actually traveling, caused by fitting tires with a different overall diameter than the ones your speedometer was calibrated for. Your speedometer doesn't measure ground speed directly — it counts how fast the wheels rotate and multiplies by an assumed distance per revolution. Change the tire diameter and that assumption breaks, so the reading drifts from reality even though the sensor is working perfectly.
Because the wheel turns fewer times for each mile on a taller tire, the dash shows a number that is lower than your true speed. Fit a shorter tire and the opposite happens: more revolutions per mile, so the speedometer reads high. The size of the gap scales directly with the percentage change in diameter.
Worked example: 225/45R17 → 245/40R18
Old diameter = 17 + 2 × (225 × 0.45 ÷ 25.4) = 24.97 in. New diameter = 18 + 2 × (245 × 0.40 ÷ 25.4) = 25.72 in. That is an error of (25.72 − 24.97) ÷ 24.97 × 100 ≈ +3.0%. So when the speedometer shows 60 mph, your true speed is 60 × (25.72 ÷ 24.97) ≈ 61.8 mph — you're moving faster than the dash claims. Over 100 true miles the odometer logs only about 97.1 miles.
Indicated vs true speed (225/45R17 → 245/40R18)
| Indicated | True speed | Difference |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 mph |
| 60 mph | 61.8 mph | +1.8 mph |
| 75 mph | 77.2 mph | +2.2 mph |
Reading the direction correctly
The sign tells you which way you are off. A positive error means the new tire is larger, so true speed is higher than indicated and the odometer under-counts; a negative error means a smaller tire that reads fast and over-counts. To work out either diameter from scratch, start with the tire size calculator, and to weigh a candidate size against your stock fitment side by side use the tire size comparison tool.
Frequently asked questions
Why do bigger tires make the speedo read low?
A taller tire covers more ground per turn, so the wheel rotates fewer times per mile. The speedometer counts rotations, so it shows a number below your true speed.
How much error is acceptable?
Stay within about 3% of stock overall diameter. Under that, the speedometer, odometer, and ABS remain close enough for everyday driving.
Does it affect the odometer and mileage?
Yes — a larger tire under-counts. At +3% you log roughly 97 miles for every 100 you actually drive, slightly slowing recorded mileage.
Can a dealer recalibrate it?
Often yes. Many vehicles let a dealer or tuner enter the new tire size or revs-per-mile; some need an aftermarket recalibration module.
Do smaller tires read fast?
Yes — fewer ground inches per revolution means more revs per mile, so the speedometer reads high and the odometer over-counts.
Is the error legal or safe?
A small error within ~3% is generally fine, but a big one can mean unknowingly speeding. Check true speed with GPS and recalibrate if it's well off.
Tire dimensions follow the standard P-metric system used across the industry — see Tire Rack's tire-size reference. The 25.4 mm-per-inch conversion is exact.
Last reviewed June 2026