Wheel Horsepower Calculator
Estimate how much engine power reaches the wheels, or back out crank horsepower from a chassis dyno number.
How to use the Wheel Horsepower Calculator
- Choose the convert.
- Enter the horsepower.
- Choose the drivetrain.
- Enter the drivetrain loss.
- Press Calculate. The result updates automatically as you type.
About the Wheel Horsepower Calculator
Manufacturers rate engines at the crankshaft, but a chassis dyno measures power at the tires. In between, the transmission, driveshaft, differential, axles and wheel bearings all absorb some power as friction and heat. The percentages used here are common rules of thumb; real losses vary with the specific gearbox, tire size and even the type of dyno, so treat the result as an estimate.
Some tuners argue that losses are partly a fixed amount of horsepower rather than a pure percentage, which means high-power cars lose a smaller share. For comparing modifications, the most useful approach is to test on the same dyno before and after, and compare wheel numbers directly.
Formula
Wheel hp = Crank hp × (1 − Loss %)
Crank hp = Wheel hp ÷ (1 − Loss %)
Note that you divide, not multiply, when going from wheel to crank. Example: 300 crank hp with 15% loss is 300 × 0.85 = 255 whp; 255 whp ÷ 0.85 = 300 crank hp.
Example calculation
Using the default values:
- Convert = Crank hp → wheel hp
- Horsepower = 300 hp
- Drivetrain = Rear-wheel drive, manual (~15%)
the calculator returns:
- Result: 255 hp
- Result in kilowatts: 190.2 kW
- Power lost in the drivetrain: 45 hp
Frequently asked questions
How much horsepower is lost through the drivetrain?
Commonly quoted ranges are 10–15% for front-wheel drive, 15–20% for rear-wheel drive and 20–25% for all-wheel drive.
Why do dyno results differ between shops?
Different dyno types (inertia vs. load-bearing), correction factors, tire pressure and strapping all change the reading. Dynojet units typically read higher than Mustang dynos.
Do automatic transmissions lose more power?
Traditional torque-converter automatics usually lose a few percent more than manuals, although modern lock-up converters and dual-clutch gearboxes narrow the gap.
Last updated: · Category: Automotive calculators