Formula in that file: "=(1+((100*D2/Q2)-100)*(R2/1000))*(D2/100)"

D2 is max vehicle speed, Q2 the required speed of the speedbonus at vehicle introduction, and R2 the given bonus.

To go through it step by step:

Compare the speed you can reach with the speed you should reach.

Multiply by a hundred and subtract a hundred. Therefore, you get a negative value if you don't reach the required speed.

Multiply by the bonus value in promille. Therefore, the higher the bonus, the more effect.

Add one. This represents the base value that gets altered by the speedbonus.

- so far, so good.

But why multiply by the vehicles speed/100? That would mean even if the vehicle goes faster than the speedbonus requires, as long as it goes under 100km/h, it might still gain less than the base value...