s12 Air drag.

[s13eater]

i think its around 0.34. yeah just checked it is 0.34.

[30psi]

there is rolling resistance to consider as well

[ChEsTeR]

The roling resistance and and transmition loses can also be imputed. I have print out from my las dyno and it shows haw much HP it looses at certen rmps. It will not be as acurate as a normal dyno du to changable wind and weight of the car but it can show at leest a general number and the graps of the torque and HP.