Much confusion still exists even among car enthusiasts about the difference between net and gross horsepower. Gross and net horsepower ratings measure the same thing, but in very different ways.
Up until 1972 horsepower was measured with an engine running no peripheral systems (AC, power steering, alternator,), no muffler attached to the exhaust, and other factors to optimize engine output.
Pressure from safety advocates and insurance companies started to mount on the automotive industry to tone down horsepower. Emission standards changing due to the switch from leaded to unleaded gasoline also caused the change from gross to net horsepower ratings.
Net horsepower ratings measured output on a car engine with all peripherals attached, including smog control systems. Net horsepower was the standard for the automotive industry until 2005.
Before the switch from gross to net horsepower ratings, it was not uncommon for family sedans to be rated at 300 horsepower. After the switch, many of the same model vehicles saw a significant drop in horsepower rating with little to no differences in the engine of the vehicle.
Despite common belief, net horsepower is not the power of the vehicle at the wheels. Wheel horsepower is the term used to measure the power output of a vehicle that actually reaches the vehicle's wheels.
