Dyno & Performance
Wheel HP ↔ Crank HP & Power-to-Weight
Convert wheel horsepower to crank horsepower (or back) with FWD, RWD, and AWD drivetrain-loss estimates you can override, then see hp per ton, lb/hp, and kg/hp.
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How to Use This Tool
- Pick the direction: crank → wheel (from a factory rating) or wheel → crank (from a dyno sheet).
- Select FWD, RWD, or AWD to load a typical drivetrain-loss estimate, or type your own percentage.
- Enter the horsepower figure and the vehicle weight in pounds or kilograms (include the driver for track comparisons).
- Read the converted horsepower and power-to-weight figures, then post the real dyno numbers on your garage.
The Formula & Physics Behind the Math
The calculator models drivetrain loss as a percentage of crank horsepower: wheel HP = crank HP × (1 − loss), and crank HP = wheel HP ÷ (1 − loss). Typical estimates are about 10% for FWD (8–12%), 15% for RWD (12–18%), and 20% for AWD (17–25%), but real losses depend on the transmission, differential, tires, and dyno type — part of the loss is fixed rather than proportional, and inertia dynos read differently from load-bearing dynos. Power-to-weight divides horsepower by mass: hp per ton = hp ÷ (lb ÷ 2,000), hp per tonne = hp ÷ (kg ÷ 1,000), and lb/hp or kg/hp is the inverse.
Frequently Asked Questions
How much horsepower is lost through the drivetrain?
Common rules of thumb are roughly 10% for front-wheel drive, 15% for rear-wheel drive, and 20% for all-wheel drive, with automatics and heavy AWD systems at the high end. These are estimates — the only way to know your number is to dyno the car.
Why doesn't my dyno number match the factory rating minus 15%?
Different dynos read differently (inertia dynos usually read higher than eddy-current load dynos), correction standards differ, and weather, heat soak, and tire pressure all move the result. Compare runs on the same dyno on similar days.
Is lb/hp or hp/ton better for comparing cars?
They carry the same information. Lower lb/hp is quicker, higher hp per ton is quicker. Drag racers often quote lb/hp; European magazines use hp per tonne. Use wheel horsepower and race weight with the driver for the most realistic comparison.
Should I post crank or wheel horsepower on my build?
Post what you measured. A dyno sheet gives wheel horsepower — label it WHP with the dyno type. If you only have a factory or tuner estimate, label it as crank or estimated so other owners can compare fairly.