This wheel horsepower (WHP) calculator estimates how much of your engine's crank horsepower actually reaches the pavement after drivetrain losses. Enter flywheel/crank HP and your drivetrain type to see the wheel power a chassis dyno would read — the number that really determines how your car accelerates.
Wheel Horsepower Formula
Wheel horsepower is the power measured at the driven wheels on a chassis dynamometer, after the engine's crank output has passed through the transmission, driveshaft, differential, and axles. Each of those components has friction and inertia that bleed off a slice of power — collectively called drivetrain loss or parasitic loss. WHP is always lower than crank HP, and it's the more honest predictor of real-world acceleration because it's what the tires get.
WHP vs Crank HP: Which Should You Quote?
Manufacturers advertise crank (flywheel) horsepower measured on an engine dyno. Tuners and enthusiasts usually talk in wheel horsepower because that's what a chassis dyno reads. Comparing the two without accounting for drivetrain loss leads to confusion — a "330 WHP" car and a "400 crank HP" car can be the same vehicle. To go from wheel back to crank, use our flywheel horsepower calculator, which divides WHP by the efficiency factor.
Drivetrain Loss by Layout
The number of components in the power path determines the loss. Manuals are typically more efficient than automatics, and AWD loses the most because it drives both axles.
| Drivetrain | Typical loss | WHP from 400 crank HP |
|---|---|---|
| FWD manual | 10–12% | 352–360 |
| RWD manual | 15–17% | 332–340 |
| RWD automatic | 17–20% | 320–332 |
| AWD | 20–25% | 300–320 |
These percentages are estimates, not fixed laws. Real loss shifts with transmission type, gear oil viscosity, tire size, ambient temperature, and even the specific dyno used — which is why the same car can read slightly different WHP on different days or machines.
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Frequently Asked Questions
Crank HP is measured directly at the engine output shaft (what manufacturers advertise). WHP is measured at the wheels after the power has traveled through the transmission, driveshaft, and axles.
FWD is generally more efficient (10-15% loss) because it has fewer components and no long driveshaft compared to RWD (15-18% loss).
Wheel horsepower is the power actually delivered to the ground, measured on a chassis dyno after drivetrain losses. It's lower than the engine's crank horsepower.
Roughly 10% for front-wheel drive, 15% for rear-wheel drive, and 20–25% for all-wheel drive, with automatics generally losing a little more than manuals.
Advertised figures are crank horsepower; a chassis dyno reads wheel horsepower after drivetrain losses, so a lower number is normal and expected.