Conversion from gross HP on an engine dyno, net HP on an engine dyno with all accessories and exhaust as installed in the vehicle and wheel HP as determined on a chassis dyno. Friction in the drivetrain and tires on the dyno rollers drop the seen reading. More or less a standard, but some drivetrains will show more or less reduction. An example is the Ford 9" as compared to the 8.8" differentials. It is accepted the 9" is stronger but has more frictional loss.
Then there is net power. When the General came out with the rounded Caprice body style that sold well to police forces, they came with the LT1 Optispark engine. In a Corvette it was 300 HP. In a Camaro it was rated at 275HP. In the Caprice the rating dropped to 250HP. Pretty much most of the difference was in the exhaust system. Corvette owners would be more tollerant of more exhaust noise. The General did work to keep the sound level down. Camaro owners take in a more broad spectrum with more wannting a fairly quiet exhaust. Then you get to Caprice purchasers that are looking for Cadillac quiet.
When you use a hub dyno tire friction is taken out as a factor.
Maybe 20% compensation is a bit high, and 15% may be closer. Without pulling the engine and running it on a dyno we can not know the exact drivetrain loses.
