How this instrument works
An engine's rated horsepower — crank horsepower, sometimes called flywheel or brake horsepower — is measured right at the engine, before any of that power has to travel through a transmission, driveshaft, differential, and axles to actually turn the wheels. Every one of those components introduces friction, and friction eats power. Wheel horsepower (WHP) is what's left after all of that loss, and it's the number a dynamometer measures when a car is strapped down and run on rollers.
The relationship is a single multiplication: wheel horsepower equals crank horsepower times drivetrain efficiency, where efficiency is whatever fraction of crank power survives the trip to the wheels. Front-wheel-drive drivetrains tend to have the shortest, most efficient path and the least loss; all-wheel-drive drivetrains, with more gears, differentials, and driveshafts in the power path, tend to lose the most. There's no single universal efficiency number — it depends on the specific drivetrain layout, gearing, and mechanical condition of the vehicle.
This gap between crank and wheel numbers is exactly why a car's advertised horsepower and its dyno-measured wheel horsepower are two genuinely different figures, and why comparing a dyno result against a manufacturer's spec sheet without accounting for drivetrain loss will make an engine look weaker than it actually is.
- Enter your engine's crank (flywheel) horsepower — the manufacturer-rated figure.
- Enter a drivetrain efficiency percentage — lower for AWD, higher for FWD, based on your vehicle's known or estimated loss.
- Read wheel horsepower — the power actually available at the wheels.
- Read drivetrain loss in horsepower — the gap between crank HP and WHP, useful for sanity-checking a dyno result against a spec sheet.
Worked example — a 660 hp RWD car on a dyno
A rear-wheel-drive car rated at 660 hp at the crank is measured on a dyno. Using an 81.73% drivetrain efficiency (a commonly cited figure for a typical RWD layout): wheel horsepower = 660 x 0.8173 = 539.4 whp. Drivetrain loss = 660 - 539.4 = 120.6 hp — the power spent turning the transmission, driveshaft, and rear differential rather than the tires.
A front-wheel-drive car, with a shorter and generally more efficient power path, tends to lose noticeably less: a 400 hp FWD car at 90% efficiency nets 400 x 0.90 = 360.0 whp, a smaller proportional loss than the RWD example despite starting from a lower crank figure.
Questions
Why is wheel horsepower always lower than crank horsepower?
Because power has to physically travel through mechanical components — a transmission, a driveshaft, a differential, sometimes a transfer case — between the engine's crankshaft and the wheels, and every one of those components has some internal friction that converts a portion of that power into heat instead of motion. Wheel horsepower measures what's left after all of that loss, which is always less than what the engine originally produced.
What's a typical drivetrain efficiency percentage?
It varies by drivetrain layout and isn't a fixed constant: front-wheel-drive cars, with the shortest power path, often see efficiencies in the high 80s to low 90s percent; rear-wheel-drive cars are commonly cited in the low-to-mid 80s; all-wheel-drive cars, with the most drivetrain components in the power path, often run lower still, sometimes into the 70s. These are general tendencies, not fixed numbers — actual efficiency depends on the specific vehicle.
Why do manufacturer specs and dyno results disagree?
Manufacturer specs report crank horsepower, measured directly at the engine under controlled conditions; a dyno measures wheel horsepower, after drivetrain loss. A car with a manufacturer-rated 300 crank hp will reasonably show something noticeably lower on a wheel dyno — that gap isn't a discrepancy or a sign of a weak engine, it's simply two different measurement points in the same power path.
Can I use this calculator to estimate crank horsepower from a dyno result?
Not directly with this version — it goes from crank horsepower to wheel horsepower, not the reverse. To go backward, divide your dyno's wheel horsepower reading by the efficiency percentage (as a decimal) instead of multiplying: crank HP is roughly wheel HP / efficiency.
Does a higher drivetrain efficiency mean a 'better' car?
Not on its own — efficiency here just describes how much of the engine's power reaches the wheels, not the car's overall capability or design quality. All-wheel-drive systems sacrifice some drivetrain efficiency in exchange for the traction and handling benefits of powering all four wheels, which is a deliberate engineering tradeoff rather than a flaw.