Officially, the M5's 4.4-liter V8 is rated at 560 horsepower and 500 pound-feet of torque, but the dyno graph reveals numbers at the rear wheels to be 527 hp and 476 lb-ft in completely stock form. While the idea of the engine being underrated is just an assumption, the general rule of thumb is a 15-percent drivetrain loss – if accurate, the new M5's losses would be just six percent. Using the 15-percent rule, the 2013 M5 would lay down 476 hp at the wheels.
The idea of BMW underrating its engine outputs is nothing new. Earlier this year, the new 328i show some impressive numbers of its own.