All cars' speedometers never show the exact speed of the car; they always show more. This is done either intentionally by the makers for safety reasons and/or are due to normal measuring system errors. For non-exotic & dream cars (ie our cars) the error is usually about +10% for a speed around 100kph (62mph). This means for example that if you read 120kpm on the speedometer, you are actually doing a bit less than 110kph. Generally, the error is linear with the speed of the car but it could also be arbitary. It could get up to 20% or more for speeds close to 200kph. There are also a few cases where the speedometer shows less than the actual speed! As far as I know, all of the above do not apply for motorcycles.

This applet will calculate the real & exact speed of your car (in any gear) given some numbers which you can get easily from the owner's manual. You will need the gear ratio for the differential and for each of the 5 gears. This is a decimal number with usually no more than 5 digits, e.g. '3.158'. If the manual has it as a fraction and the numerator is 1 (e.g. '1/3.158') you will need only the denominator. If it's a general fraction (e.g. '32/45') put in the result of the division.

You will also need the diameter of the front tyre (if the car is a FWD) or the rear tyre (if it's a RWD). For a given tyre dimension (e.g. 165/60/14) the diameter can vary between different manufacturers. For the exact number you can ask your local dealer or try the web, otherwise take the number from the generic diameter list provided here. (Values are in millimeters).

Press the "Example" button to see an example set of values (those of a Porsche 911 Carrera-2 (964) 3.6 on Continental 225/50/16).

After completing the above, just type in the engine's RPM you want and hit the "Calculate" button. The formula used calculates the exact RPM that the driving wheel is revolving and then, given the tyre diameter, calculates the exact speed. We of course assume that the tyres are properly inflated and the wheels are not spinning.

If you have a 6th gear or you want to know about the reverse too, then you can fill-in any of the 5 gear fields with the 6th and the reverse and get the speed from the respective output. But remember not to leave the rest of the input fields blank/zero or you'll get an error message.