Conventional wisdom from other forums is RWHP = FWHP*.85 for manual transmissions and RWHP = FWHP*.80 for automatic trans. Most data came from chassis dyno operators comparing RWHP with the manufacturer's engine rating. Maybe a couple tuners would run an engine on an engine dyno, then a chassis dyno. Various brands of chassis dynos would give different HP numbers for the same vehicle. Chassis dynos calculate torque and HP by measuring incremental angular accelerations of the big drum. Its inertia "loads" the engine and goes into the calculations. Unknown and variable quantities of inertia come from the wheels, engine flywheel, transmission etc that contibute some inertia to the whole system aft of the engine. If one really wanted to see large chassis dyno numbers, he'd use an aluminum flywheel and the lightest wheels and tires he could find. The engine would make the same torque and HP, but the chassis numbers would be higher. I believe that's how it works.
