Oh dear, someone mistook Top Gear for real information.
Horsepower figures from manufacturers are rated at the
flywheel (read: Engine). A dyno test measures horsepower at the actual
wheels.
What the engine is always greater than what is actually put down on the road. It takes power to accelerate the transmission, driveshaft, differential, wheels, and all other rotating mass. It is physically impossible for a car to put down as much as its engine makes. For instance, a 1000hp Bugatti Veyron actually puts down closer to 800bhp
to the wheels due to drivetrain loss.
Top Gear dynoed a 2007 GT500 at 447
bhp. With a FR layout car, you can expect around a 15% loss of power to the drivetrain.
The GT500 is rated from the factory at 500hp (not bhp, hp). This means Ford says the engine is producing 500hp. To find the expected power the wheels actually put down we can use simple math.
500hp*.15=75
500hp-75=
425bhp
To find the drivetrain loss evident from the Top Gear test.
447/500 * 100 = 89.4%
100-89.4=
10.6% drivetrain loss
This means that in reality either the GT500 only lost about 10.5% of its power to the drivetrain or Ford actually underrated the GT500.
Top Gear attempted to make the GT500 look like an overrated car, so that's why they decided to leave the little (absolutely crucial) detail about drivetrain loss out of their test. In other words,
Top Gear bends the truth.
The 2013 GT500 has dynoed at 600bhp. This is on the money for the 662hp claimed by Ford.
Research before you believe a single word that Top Gear says. 👍