" Gross horsepower is a measurement of engine output, taken at the flywheel, without the engine installed in a vehicle. Since the engine has no load on it, all of its energy can be used for making horsepower.
Wheel-driven horsepower, by comparison, is a measurement taken at the driven wheels of a vehicle on what's called a dynamometer. This is done by placing the vehicle's driven wheels on a large roller and accelerating the wheels up to redline in first or second gear. The vehicle's ability to turn this roller is measured and calculated to come up with a figure that represents how much horsepower is actually available to move the vehicle around -- or real-world horsepower....."
http://www.edmunds.com/car-technolog...eal-world.html