Wheel Horsepower Calculator
Convert crank (brake) horsepower to wheel horsepower by subtracting drivetrain loss, and see how many horses the driveline eats.
How it works
Wheel hp = crank hp × (1 − drivetrain loss%) · Power lost = crank hp − wheel hp
Manufacturers rate engines at the crankshaft (brake horsepower), but a dyno measures power at the wheels, which is lower because the clutch or torque converter, gearbox, driveshaft and differential each consume some power as friction and heat. Front-wheel-drive cars have the shortest driveline and lose the least (around 10–12%), rear-wheel drive a bit more (~15%), and all-wheel drive the most (often 20–25%) because power passes through extra gears and a transfer case. These are rule-of-thumb figures — a well-built manual can lose less, an automatic or a lossy AWD system more.
Worked example
A 300 hp rear-wheel-drive car with a 15% driveline loss delivers 300 × (1 − 0.15) = 255 whp, meaning about 45 hp is absorbed getting from the flywheel to the pavement.
Frequently asked questions
What is a typical drivetrain loss?
As a rough guide, front-wheel drive loses about 10–12%, rear-wheel drive around 15%, and all-wheel drive roughly 20–25%. The exact figure depends on the transmission type, gearing and how the dyno measures, so treat these as estimates rather than exact constants.
Why is wheel horsepower lower than crank horsepower?
Every rotating component between the engine and the tyres — clutch or torque converter, gearbox, driveshaft, differential and even the wheels and tyres — soaks up power through friction, windage and heat. What reaches the road is always less than what the engine produces at the flywheel.
Can I estimate crank power from a dyno number?
Yes — divide the measured wheel horsepower by (1 − loss%). Because the loss percentage is only an estimate, the back-calculated crank figure is approximate, which is one reason enthusiasts usually compare wheel horsepower directly.
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Educational estimate — real figures vary by vehicle, load, conditions and manufacturer specs. Always follow your owner’s manual.