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How fast is a car really?

It is a law that a car's speedometer has to run ahead, to show a higher speed than the actual driven one. An 850CSi for example reaches 200 kph only when the speedometer is already at 209.

But there is another possibility, apart from enabling the service menu of the onboard computer, to calculate the real speed and the mechanical top-speed (without air- and roll resistance and electronic limiters). The formula is quite simple: Engine rpm / gear ratio / final drive ratio × tyre circumference.

This only gets complicated because of tyre circumference where the tyre slip has to be taken into account. The little program GearSpeed can be helpful there. Here you can input all relevant data and then read the speed in the gears. The Windows-software offers, apart fom E31-specific presets, manual input so that it can be used with any other car as well.

Click here to download (70 KB)