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.
The issue for six sigma fans is the calibration and guage R&R of the dynamometer. Different rolling roads will give different results but results on the same unit are comparible. So the Morganeers +4 and Richard V6s roadster plus Belle all run on the same rolling road give comparible resluts but the absolute numbers on a different rolling road could / would be different.
