The term horsepower refers to the work power produced by cars or equipment. Car enthusiasts and manufacturers have two ways of measuring horsepower, at the wheel or at the crankshaft. Each method will give different results, so it is important to understand the differences.
According to Merriam-Webster, horsepower is "a unit of power equal in the United States to 746 watts and nearly equivalent to the English gravitational unit of the same name that equals 550 foot-pounds of work per second." The last part refers to horsepower as it was measured before cars were invented. Using pulleys, one horsepower was the ability to lift 550 pounds, one foot off the ground, in one second.
Crankshaft horsepower is measured with only the accessories needed for the engine to run, and is measured at the flywheel.
Wheel horsepower is measured at the drive wheels, with the engine installed in the vehicle and all accessories in place.
The exact same car will register lower wheel horsepower than crankshaft horsepower This is because for wheel horsepower, the turning force generated by the engine must travel through the transmission, the drive train, and turn accessories, such as the alternator and power steering. These things all contribute to parasitic loss of power. By the time the turning force gets to the rear wheels, some has been lost due to these factors.
