It is neither an equation of the type
X = Y * Z
nor an
X = Y + Z

(x= bhp @ wheels, y=bhp @ fly, z = the "answer" people are looking for)

its far more complex than that, including adders, multipliers and powers, some relating to the box itself and others to the power of the engine, not to mention the rpm of the engine too, and specifically the gear you are in as well.


If you want a total ballpark for a 4wd cossie, try something along the lines of 20bhp + 15%, its likely to be closer than anything quoted in this thread, but still falls under the "complete and utter over simplified bollocks" category at the end of the day.