Rear Wheel Horsepower to Flywheel Horsepower Conversion
Convert rear wheel figures to estimate flywheel figures.
Flywheel Horsepower = FHP; Rear wheel Horsepower = RHP
FHP = RHP + 17% (auto trans) 
FHP = RHP + 25% (man trans) 
RHP = FHP - 15% (auto trans) 
RHP = FHP - 20% (man trans) 
Example: 312.25 FHP = 249.8 RHP + 62.45 HP*
* Manual trans: +25%
http://www.cobranet.com/calcs.htm