I was searching for a formula - haven't managed to find a thread on here about it but I'm sure someone will prove me wrong.

Anyway I found this:

"Converting wheel bhp to flywheel bhp and vice versa

To reflect the fact that % losses are high for low powered cars and vice versa I use the following equations which have been found to correlate well with real world transmission losses.

FWD cars - add 10 bhp to the wheel figure and divide the result by 0.9

RWD cars - add 10 bhp to the wheel figure and divide the result by 0.88

4WD cars - add 10bhp to the wheel figure and divide the result by 0.84

To estimate the expected wheel bhp from a known flywheel bhp just reverse the equations

FWD - multiply flywheel power by 0.9 and then deduct a further 10 bhp

RWD - multiply flywheel power by 0.88 and then deduct a further 10 bhp

4WD - multiply flywheel power by 0.84 and then deduct a further 10 bhp

Remember, these percentages are not "gospel" - they are good realistic averages."

The rest of article is at Puma Race Engines Technical Guide - Measuring Engine power - engine dynos and rolling road dynos

very interesting... I know at the 2 RR's I've been to my car has given 147bhp at the wheels (the same as some remapped fabia vRS's) the vRS's gave approx 170-175 bhp at the fly, but mine only gave 158 at the fly? why has mine SOOO much less drag than a vRS? both FWD both 1.9 diesel, both 6 speed ect? given the maths above mine would result in 174bhp at the wheels, which seems more likley given its performance on the road against a mapped vRS....

They're figures based on research for a Ford Puma... doesn't mean they apply to all cars.

Is their a formula for working out transmission loss? Surely every car whether be make or model be different?

There's not one that's entirely accurate.