I fudged the efficiency to fit the numbers. The factor of two is the dyno:wheel ratio, like Ken said. That should have been measured and corrected for by the dyno operator, but probably wasn't. Sorry about the mud, I was in a hurry.
I looked up the factory gear ratio on Dodge's web site and the 27% loss is a very typical number I hear from high performance guys who care about their rear wheel horsepower.
The drivetrain losses due to gears and bearings are fairly insignificant, but the loss from the rolling resistance of the tires is big. It is the dominant loss usually, followed by accesories, which are usually unconnected for the factory advertised numbers. The alternator, fan, water pump, and power steering all draw quite a bit of power. If you disconnect the accesories, overinflate the tires, and run an electric water pump on the dyno you will see a much better driveline efficiency.
