Flywheel horsepower is the measurment used by car companies. They just strap a dyno to the engines flywheel, which the the first thing the crankshaft touches.
Most people dyno their horsepower at the wheels. There is always some parasitic loss through the drivetrain to the wheels so there is less horsepower read at the wheels.
Flywheel = at the engine, more hp
Wheel = at the wheels, less hp
So an average loss is about 15% in a manual. Which means that a Camaro with 320 hp at the flywheel will have 272 at the wheels.
2002 Sebring Silver Metallic Z28