About this tool
The Speedometer Error Calculator shows how a change in tire size throws off both your speedometer and odometer. A car's speedometer counts wheel revolutions and assumes the factory tire's overall diameter. Fit a taller or shorter tire and each revolution now covers a different distance, so the needle reads a speed that no longer matches how fast you are actually moving.
Enter each tire either as a P-metric size or as a plain overall diameter in inches. From a P-metric size (width/aspect Rrim, e.g. 265/70R17) the overall diameter is D = (width_mm × aspect/100 × 2 ÷ 25.4) + rim_in — the sidewall height is the width times the aspect ratio percentage, counted twice because there is sidewall above and below the rim. Your true speed is then Actual = Indicated × (D_new / D_stock), and the reading error is (D_new − D_stock) / D_stock × 100. The same ratio scales odometer distance: a larger tire makes the odometer under-count miles.
Everything is computed locally in your browser. A taller tire makes the speedometer read low (you are going faster than it shows) and the odometer under-record distance; a shorter tire does the opposite. To keep readings accurate after a size change, the vehicle's speedometer calibration (or the tone-ring/final-drive data in the ECU) should be adjusted to the new diameter. Use this as a planning estimate — actual rolling diameter varies slightly with load, pressure, and wear.