Wheel HP vs Crank HP & Drivetrain Loss
Why the dyno number is lower than the brochure number, and typical losses for FWD, RWD and AWD.
Put a car on a chassis dyno and the horsepower number almost always comes out lower than the figure in the brochure. That is not a faulty dyno — it is the difference between crank horsepower and wheel horsepower, separated by drivetrain loss.
What the Dyno Actually Reads
A chassis dyno measures power at the wheels, because that is where the rollers contact the car. An engine dyno, by contrast, measures power right at the crankshaft with no drivetrain attached — this is the crank or flywheel horsepower, also called brake horsepower (BHP), and it is the number manufacturers advertise because it is the highest and is independent of the gearbox fitted.
Drivetrain Loss by Layout
Between the crank and the tyres, energy is lost to friction in the gearbox, the differential, the bearings, and the simple job of spinning all that rotating mass. As a rough guide:
| Drivetrain | Typical loss | Why |
|---|---|---|
| Front-wheel drive (FWD) | ~10% | Compact transaxle, short power path |
| Rear-wheel drive (RWD) | ~15% | Longer driveshaft, separate diff |
| All-wheel drive (AWD) | ~20–25% | Extra diffs, transfer case, more mass |
These are estimates — manual gearboxes generally lose less than automatics, and a worn or cold drivetrain loses more. Use the wheel horsepower calculator to convert crank to wheel HP, or the flywheel horsepower calculator to work back the other way.
How to Estimate Crank HP From a Dyno Sheet
If your RWD car puts down 300 WHP and you assume 15% loss, the crank figure is 300 ÷ (1 − 0.15) = 300 ÷ 0.85 ≈ 353 crank HP. Going the other way, a 400 crank HP RWD car should make roughly 400 × 0.85 = 340 WHP. Always treat the loss percentage as an estimate, not a constant — the only exact number is the one your specific car records on the rollers.
Frequently Asked Questions
Advertised figures are crank (flywheel) horsepower measured at the engine. Wheel horsepower is measured after losses through the transmission, driveshaft and differential, so it is typically 10–25% lower depending on drivetrain layout.
Wheel horsepower is directly measured on a chassis dyno, so it's the real-world figure. Crank horsepower is estimated from it using an assumed drivetrain-loss percentage.
Automatics generally lose slightly more than manuals because of the torque converter and pump, though modern automatics have narrowed the gap considerably.
As a rough guide, about 10% for front-wheel drive, 15% for rear-wheel drive, and 20–25% for all-wheel drive. Automatics usually lose slightly more than manuals.
Divide wheel horsepower by (1 minus the loss fraction). For example, 300 WHP with 15% loss is 300 ÷ 0.85 ≈ 353 crank HP. This is an estimate based on a typical loss percentage.