Crank vs. rear wheel
Completely agree with A-Bomb on this one.
I know two things for a fact -
1/ - Ford use 18% drivetrain losses for their new cars fitted with factory built modular V8 OHC engines. A Ford production engineeer told me this.
2/ - A well known builder down here uses a rule of thumb of 120 HP losses for a 500HP engine through a Ford 9" and 727 with a 4000 stall. He says he's seen it many times on the chassis dyno against the engine dyno figures.
This works to around 22%, which is the figure I've always used.