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Speedometer Error Calculator

Enter your indicated speed and old & new tire diameters.

Result

105.77
5.77%
5.77

How it is calculated

The speedometer counts wheel turns and assumes the original tire size, so the true speed scales with the ratio of the new overall tire diameter to the old one: true speed = indicated × (new diameter ÷ old diameter). The error is diff = true − indicated and error % = diff ÷ indicated × 100. Diameters mean the full outer diameter of the tire (rim plus both sidewalls), and the two must use the same unit, for example inches. The speed result has the same unit as the speed you enter, km/h or mph. A positive error means you are going faster than the display says; a negative one means slower. All three values must be above zero, and results are rounded to two decimals. Changing only the rim size, without the overall diameter, does not alter the result.

Worked example

The old tires were 26 inches tall and the new ones are 27.5 inches. The speedometer reads 100. The ratio is 27.5 ÷ 26 = 1.0577. True speed: 100 × 1.0577 = 105.77. The difference is 105.77 − 100 = 5.77, and the error is 5.77 ÷ 100 × 100 = 5.77%. At an indicated 100 you are really doing about 105.8 in the same speed unit, so a limit of 100 would be exceeded.

Try this example →

Common mistakes

  • Entering the rim diameter or the tire width instead of the overall diameter. Compute the full diameter from the tire size marking and use that for both tires.
  • Mixing units, such as the old tire in centimetres and the new one in inches. The ratio is then meaningless.
  • Swapping the old and new fields. A bigger new tire makes you faster than indicated; swapped, the tool shows you slower than indicated.
  • Reading the percentage as a speed. A 5.77% error is not 5.77 km/h; at higher speeds the speed difference is larger, at lower speeds smaller.
  • Assuming the result fixes every error. The tool covers only the diameter change, not the factory speedometer tolerance, tire wear or tire pressure.

About this calculator

A car's speedometer is calibrated to the original tire diameter specified by the manufacturer, since the gauge actually measures wheel rotation speed and converts it to road speed using that assumed diameter. Fitting a different-sized tire or wheel changes how far the car actually travels per rotation, so the speedometer keeps displaying speed based on the old diameter — this calculator applies the ratio of new to old tire diameter to your indicated speed to find your true road speed and the resulting error.

This calculation matters to car enthusiasts and off-road drivers who upsize or downsize wheels and tires for looks, off-road clearance, or handling changes, since driving faster than the speedometer shows can mean unknowingly exceeding a speed limit, and the same diameter shift also throws off the odometer and any speed-dependent electronics tied to wheel-speed sensors.

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Comments (1)

  • Editorial team

    The odometer counts the same wheel turns as the speedometer, so a taller tyre skews recorded distance by the same ratio. At 27 against 25, a trip of 100 real miles would show roughly 92.6 on the dial. For the diameters, measure the overall tyre height yourself where you can, and cross-check at a steady speed against a GPS reading. Tell us which tyre swap you are planning and what GPS showed.

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