I had to do some tests. Approximate car weight 1600 kg. Tests done with my Passat wagon.
Initial speed 110 kmh, final speed 100 kmh, time 7 sec, gives 25 hp for a indicated speed of 105 kmh.
Initial speed 90 kmh, final speed 80 kmh, time 9 sec, gives 15.8 hp for an indicated speed of 85 kmh...