Speedometer Error Calculator

Your speedometer works by counting wheel rotations and multiplying by the tire’s circumference.
Change the tire size and the circumference changes, but the speedometer carries on using the original figure, so the number on the dial is wrong.

Tire outer diameter (mm) = (wheel diameter in inches × 25.4) + (2 × section width × aspect ratio / 100) Tire circumference = π × outer diameter

The error: True speed = Indicated speed × (New circumference / Original circumference) Error % = (New circumference / Original circumference - 1) × 100

Notice that π cancels out of that ratio. Comparing circumferences and comparing diameters give exactly the same percentage, which is why our tire size comparison page can report the same error working from diameter alone.

If the new tire is larger, each rotation covers more ground. The speedometer counts the same rotations and so understates how far you have gone, meaning your true speed is higher than indicated. That is the direction that collects speeding tickets.

Example: switching from 215/65R15 to 235/65R15:

• Original OD: (15 × 25.4) + (2 × 215 × 0.65) = 381 + 279.5 = 660.5 mm
• New OD: (15 × 25.4) + (2 × 235 × 0.65) = 381 + 305.5 = 686.5 mm
• At an indicated 60 mph, true speed = 60 × (686.5 / 660.5) = 62.4 mph, an error of +3.9%

That example is worth a second look, because it is a width change of only 20 mm and it still lands just outside the ±3% window that tire fitment guides treat as the safe range. Aspect ratio is the multiplier here: at 65 series, 20 mm of extra width buys 13 mm of extra sidewall on each side.

Manufacturers deliberately calibrate speedometers to read slightly high, and most OEM units read 2 to 4% above true speed on the original tire. A tire change that adds another 5% on top of that baseline can leave you 3 to 4 mph over on a 70 mph freeway while the dial still looks legal.

Vehicles with TPMS (Tire Pressure Monitoring System) may also need a reset after a significant size change, and on many cars the ABS and traction control take their wheel speed from the same source, so they inherit the error too.