How to Calculate and Convert Rear Wheel Horsepower

January 27, 2012

Many muscle car enthusiasts are always looking for a way to calculate rear wheel horsepower on their successively modified vehicles. Rear wheel horsepower is the amount of power that actually makes it from the engine through the drivetrain and to the wheels of the vehicle. It's a useful indicator of performance that will allow people to make informed purchasing decisions and modifications as they compare several different cars and trucks. Unfortunately the conversion and calculation of rear wheel horsepower is not as intuitive as you might think. It's best to take into consideration the crank horsepower and type of drivetrain before making an estimate of whp, or wheel horsepower. In order to make an informed estimate, a simple formula may be employed. Just follow the steps below in order to calculate rear whp for most any vehicle.

1. Determine Base Power of the Vehicle - Determine the base power of the vehicle, either by having a dyno performed or by obtaining the engine horsepower amount from the manufacturer (normally displayed as crank horsepower).
2. Multiply Base Horsepower by the Appropriate Percentage - Automatic transmissions are said to lose 20% of the power from the engine to the wheels, while manual transmissions lose 15%. To determine rear whp, multiply the base horsepower by 0.80 if you have an automatic transmission and 0.85 for a manual transmission. Example: 250 hp x 0.80 = 200 rear whp.
3. Convert Horsepower to Other Measures - 1 horsepower = 550 foot-pounds/second, 33,000 foot-pounds/minute, 42.44 Btu/minute, and .7456999 * kilowatts. Therefore depending on what you are trying to convert horsepower to, multiply the total from Step 2 by the appropriate measure. Example: 200whp x 550 foot-pounds/second = 110,000 foot-pounds/second.

Rear wheel is fairly easy to calculate as long as you keep the above steps in mind.