Wheel HP vs Crank HP & Drivetrain Loss

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 tires, 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:

These are estimates — manual gearboxes generally lose less than automatics, and a worn or cold drivetrain loses more. Our calculators default to 11% FWD, 15% RWD and 23% AWD, the middle of these ranges. Use the HP to WHP calculator to convert crank to wheel HP, or the WHP to HP 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.

Five Reasons a Dyno Reads Lower Than Expected

When a healthy car shows a disappointing number, drivetrain loss is only one suspect. Before assuming anything is wrong with the engine, rule these out:

1. The correction standard. The same pull corrected to SAE J1349 reads several percent lower than one corrected to the older "STD" standard, and an uncorrected figure on a hot day lower still. Check which one is printed on the sheet.
2. The dyno type. Inertia dynos and load-bearing dynos can read the same car several percent apart. Neither is wrong; they measure differently, so compare like with like.
3. Heat soak. Back-to-back pulls heat the intake air and, on turbo cars, the intercooler. Power often falls run after run. Cool-down time between pulls matters.
4. Tires and strapping. Low tire pressure, heavy wheels or a car strapped down too tightly all add rolling resistance that the dyno counts as lost power.
5. The gear used. Pulls are normally made in the gear closest to 1:1. A pull in a lower gear spins the drivetrain faster and can read differently.

Reading the Number in a Car Advert or Forum Post

Sellers and forum posts quote both figures, often without saying which. A few habits keep comparisons honest:

• Ask where it was measured. "400 hp" from a chassis dyno is a very different car from "400 hp" on a manufacturer's spec sheet — roughly 60–90 hp different once drivetrain loss is accounted for.
• Watch the units. European figures are often PS (about 1.4% larger numbers than hp), and Australian dyno sheets use rear-wheel kilowatts (rwkw). Convert before comparing.
• Compare gains, not totals. A before-and-after pair from the same dyno on the same day is far more reliable than two totals from different shops. A "+30 whp" gain is meaningful; "made 330 here and 360 there" usually isn't.

Both directions have calculators: whp to hp for imperial dyno sheets and rwkw to hp for the rear-wheel kilowatt notation used across Australia and New Zealand.

To turn a published crank rating into an expected dyno number, use HP to WHP.