Speedometer error calculator
A speedometer counts wheel revolutions. Change the rolling circumference and every reading shifts by the same proportion.
Actual speed
103.02km/h
- Speed error
- 3.02
- Error as a percentage
- 3.015 %
- Actual distance travelled
- 10 301.5
- Odometer shortfall
- 301.5
- Indicated speed to actually do that speed
- 97.07
- Circumference ratio
- 1.030 15
Show the working
- Original rolling circumference = 1990
- New rolling circumference = 2050
- Indicated speed = 100
- Distance shown on the odometer = 10 000
- Circumference ratio = 1990 > 0 ? 2050 / 1990 : 0 = 1.030 15
- Actual speed = 100 * 1.030 15 = 103.015
- Speed error = 103.015 - 100 = 3.015 08
- Error as a percentage = (1.030 15 - 1) * 100 = 3.015 08
- Actual distance travelled = 10 000 * 1.030 15 = 10 301.5
- Odometer shortfall = 10 301.5 - 10 000 = 301.508
- Indicated speed to actually do that speed = 1.030 15 > 0 ? 100 / 1.030 15 : 0 = 97.0732
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- true speed = indicated speed x new circumference / original circumference
Most regulations require a speedometer to never under-read, so factory speedometers typically read 2 to 5% high already.
Fitting larger tyres makes the speedometer read low, which is the direction that gets people fined.