5,390 pounds does not usually sprint. Yet BMW's M5 claims 60 mph in 3.4 seconds because mass is only half the equation: its plug-in hybrid powertrain sends 717 horsepower and 738 lb-ft through all four wheels. This is no featherweight trick. It is a traction calculation, backed by costly hardware.The electric motor supplies the ambush. Housed in the eight-speed transmission, it delivers torque bef...More