I seem to remember Ecutek quoting transmission losses of circa 18% when referring to their road dyno software.

My take on it is that It's never going to be an accurate figure as there are way too many variables. On a 500bhp car 18- 20% seems to be about right. It's a guideline. It's never going to be 10% and it's never going to be 30% but it will give you an idea when you compare the bhp and whp. I'll ask for both when I get mine done and work out the losses from there.

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605whp
Quarter Mile in 10.9 seconds @ 135mph /217kph
0-100 kph 3.2 seconds