I have been seeing alot of rear wheel horsepower mention in megz and other places. it always lower than the rated HP. what's the difference and how are they difference???

 

Unless you take the engine out of the car and put it on a special engine dyno, there is no way to measure flywheel HP. Thus, RWHP prevails. RWHP is the only thing (on a practical scale) that can be measured.

RWHP takes into account friction losses caused by tranny, rear end and the weight of the metal that needs to be accelerated(inertia).

 

A chassis dyno will measure exactly how much horsepower is getting to the rear wheels.
You lose some power through the drivetrain so RWHP is what is really getting to the tires.