On the rolling road they measure power at the wheels as part one of the run where the car accelrates. They then lift off the accelerator and it measures the drag using the DIN norm as the rear tyres slow the rollers.
They take the drag & add it to the power at the wheels to give you crank BHP...
Rolling road dyno flywheel figures are still a guess. Here is an explanation you will like.
https://www.linkedin.com/pulse/dyno-theory-chassis-dynamometer-road-load-equation-vibin-jacob/ Of course, as you state, if you are using a dyno for the right reasons (tuning)..such things are best ignored as a world comparison with other cars, using different dynos. .
As a matter of interest, I canvassed UK Morgan race preparers many years ago. The consensus was trad Morgans have a higher percentage of difference between flywheel and rear wheels than most cars.
But I am idly (pun!) curious. I am going to ask John Eales. He should know better than most because he uses an inhouse flywheel dyno and his racing customers often use rolling wheel dynos as well. Let's see if I get an answer. He always has been very kind and we have pals in common.
gmg
