note on rwHP v crankshaft HP

"The drivetrain efficiency is a combination of transmission, driveshaft lash and final drive gear and lash translated to the hub of the rear wheels. At peak torque (not HP) the generated torque loss at the rear wheel [on the Lightning] is 11.2%, which is way higher than typically seen on an auto. About 20% is the of the top of my head number to use for most autos."

In a later post, Gregg explained that:

"In a Lightning at max torque in 3rd gear (only way to measure it right) it's 11.2%, and I'm not even going to get into how accurate that number is unless you're an automotive or electrical engineer with experience on drivetrain testing.  Every time I mention it I get all kinds of "you're phu**ing crazy" comments, but that's the number, if you must know. It also has very little to do with the real world, just know for an automatic it's darned efficient."

Note than the 11% figure is at peak torque and may not be reflective of the powertrain losses at the horsepower peak.  However, the math does work.  380 rated crank HP X .89 = 338 rwHP, which is a pretty typical chassis dyno result.