π Wheel Horsepower Calculator
Crank HP to WHP by drivetrain, and back again.
Wheel horsepower
How to Use the WHP Calculator
Pick the direction, set your drivetrain (or type a custom loss percentage from your dyno sheets), and enter the horsepower figure. The calculator converts crank to wheel or wheel to crank and shows the power lost in the drivetrain. Use it to translate brochure numbers into reality and dyno sheets into comparisons.
Choose crank-to-wheel or wheel-to-crank.
Set drivetrain or enter your measured loss percent.
Hit Convert Power. Compare WHP per pound across cars for the real ranking.
How Drivetrain Loss Math Works
Every gear mesh, bearing, and spinning shaft converts a little power into heat and noise. The loss is modeled as a percentage because friction scales with load: double the power, roughly double the absolute loss. Working backward divides by the same factor, which is how tuners estimate crank gains from dyno-measured wheel gains.
Worked example, verified: 300 crank HP, RWD, 15% loss. WHP: 300 Γ 0.85 = 255. Lost: 45 HP to heat and noise. Reverse: a dyno reading of 255 WHP implies 255 Γ· 0.85 = 300 crank HP. AWD version: 300 Γ 0.78 = 234 WHP, which is why identical engines feel stronger in RWD cars. FWD: 300 Γ 0.90 = 270 WHP, the efficiency advantage of a short, simple drivetrain.
Where the Power Goes
The torque converter (or clutch), transmission gears, driveshaft, differential gears, and axle bearings each take a cut. Automatics traditionally lost more than manuals through converter slip, though modern lockup converters narrowed the gap. AWD adds a transfer case, extra differentials, and more spinning mass, hence the 20%+ figures. Lightweight flywheels and carbon driveshafts reduce rotational inertia, which shows up as quicker revving more than peak WHP gains. And tires matter on the dyno: underinflated or oversized tires inflate losses, which is why serious testers standardize setup before comparing.
Dyno Numbers: Reading Them Honestly
Chassis dynos measure WHP, and not all dynos agree: Dynojets read higher than Mustang dynos by 10% or more on the same car, because they measure differently (inertia vs loaded). Correction factors (SAE, STD, uncorrected) adjust for weather and can move numbers several percent. Gears, tire pressure, and even strap-down tightness shift readings. The rules: compare before-and-after on the same dyno, same day, same settings; treat cross-shop comparisons as entertainment; and remember manufacturers quote crank HP from engine dynos in ideal conditions. Your car's 300-hp brochure and its 255-WHP reality are both true, measured at different ends of the drivetrain.
Tips for Power Comparisons
Rank cars by WHP per pound, not peak HP: divide wheel horsepower by curb weight. A 255-WHP car at 3,200 lbs (12.5 lb/hp) outruns a 300-crank-HP car at 4,200 lbs all day. Mind the curve: peak WHP tells you the best moment, while the area under the torque curve tells you the whole race. Automatics with more gears keep engines in the powerband, effectively multiplying usable WHP. And when a tune claims +40 HP, ask wheel or crank; honest tuners show the dyno sheet, and the sheet speaks WHP.
Worked Example: Three Real Cars Compared
Brochure numbers hide the drivetrain tax, so run three cars through it. A 315-hp Civic Type R is front-wheel drive, about 10 percent loss: 315 x 0.90 = 283.5 WHP. A 382-hp GR Supra is rear-wheel drive, about 15 percent: 382 x 0.85 = 324.7 WHP. A 271-hp WRX is all-wheel drive, about 22 percent: 271 x 0.78 = 211.4 WHP. The Supra's brochure lead of 67 hp over the Type R shrinks to 41 at the wheels, and the WRX gives up 60 hp to its own drivetrain.
Now divide by weight for the ranking that matters. At roughly 3,200 lb, the Type R carries 3200 / 283.5 = 11.3 lb per wheel horsepower. The Supra at roughly 3,400 lb: 3400 / 324.7 = 10.5. The WRX at roughly 3,400 lb: 3400 / 211.4 = 16.1. WHP per pound predicts real acceleration far better than crank horsepower ever will, which is why a 300-hp featherweight embarrasses 400-hp barges at the drag strip.
Percentage vs. Fixed Loss: Which Model Wins?
This calculator uses the percentage model: loss scales with power. Some dyno shops use a fixed model instead, adding a flat number like 40 hp regardless of output. Both are approximations, and the truth sits between them. Bearing drag, oil pumping, and seal friction are mostly fixed costs of spinning the drivetrain. Gear mesh friction and tire flex grow with the torque passing through them.
Use the percentage model for comparing stock cars, which is what most people need. It is simple and close enough. Switch to fixed or measured loss when evaluating modifications on one car: adding 100 crank horsepower does not add 15 wheel horsepower of loss, it adds a few, because the fixed costs were already paid. That is why bolt-on gains often over-deliver on the dyno versus percentage math. Either way, never compare a Dynojet WHP number against a Mustang-dyno WHP number directly; the dynos themselves disagree by 10 percent or more.
The AWD Tax in Real Numbers
Take a 400-horsepower engine and bolt it to three different drivetrains. Front-wheel drive at 10% loss: 400 x 0.90 = 360 wheel horsepower. Rear-wheel drive at 15%: 400 x 0.85 = 340. All-wheel drive at 22%: 400 x 0.78 = 312. Same engine, same fuel, a 48-horsepower spread from the crank to the tires, purely from how many differentials and driveshafts the power crosses. This is why AWD cars need bigger engines to match their RWD rivals in a straight line, and why they make the power back in the first 60 feet where traction rules.
The tax is not constant, which is the argument against the percentage model. At light cruise, drivetrain losses are closer to fixed: spinning the gears and churning the oil costs roughly the same whether the engine makes 100 or 400 horsepower. The percentage model fits wide-open throttle; the fixed-loss model fits the highway. For comparing dyno sheets and bench racing, percentages are the convention and they are close enough. For engineering the drivetrain itself, the fixed losses are where the efficiency gains hide.
Dyno Types: Why Numbers Differ
Two dynos can read the same car 10 to 15% apart, and both are "correct." A Dynojet measures how fast the car accelerates a heavy drum (inertia), while a Mustang dyno applies an electromagnetic load to simulate real road resistance (loaded). Inertia dynos typically read higher, loaded dynos lower, and neither is wrong; they are measuring different things. This is why forum arguments about "my car made 350" are meaningless without the dyno type attached.
The rule tuners live by: same dyno, same day, same conditions for before-and-after comparisons. A baseline pull and a tuned pull an hour apart on one dyno tell you exactly what the tune gained, whatever the absolute number says. Chasing numbers across dynos, days, and weather is astrology with a printout.
Frequently Asked Questions
How do you convert crank HP to wheel HP?
Multiply by one minus the drivetrain loss: a 300-hp RWD car at 15% loss makes about 255 WHP. Select your drivetrain above; the calculator handles both directions, WHP back to crank too.
What is the difference between HP and WHP?
Crank (brake) horsepower is measured at the engine's flywheel on an engine dyno; wheel horsepower is measured at the wheels on a chassis dyno. The drivetrain (transmission, driveshaft, differential, axles) eats 10 to 25% between the two. WHP is what actually moves the car.
How much drivetrain loss does AWD have?
Typically 20 to 25%, versus about 15% for RWD and 10% for FWD. More differentials, more shafts, more loss. Modern AWD systems with efficient designs trend toward the low end of that range.
Why do dyno numbers vary between shops?
Dyno type matters most: Dynojets famously read 10 to 15% higher than Mustang or Dyno Dynamics dynos. Correction factors, weather, gear choice, and tire setup all move the number. Compare deltas on the same dyno, not absolutes across shops.
Is WHP or crank HP used for 0-60 estimates?
WHP, plus weight. Power-to-weight at the wheels determines acceleration: a 3,000-lb car with 255 WHP (11.8 lb/hp) will run with heavier cars carrying bigger crank numbers. Magazine test figures reflect WHP realities, whatever the brochure claims.
Do electric cars have drivetrain loss?
Less: single-speed gearboxes and no torque converter mean EV drivetrain losses run roughly 5 to 10%. That is one reason EVs feel stronger than their rated power suggests; more of it reaches the wheels, instantly.
How can I measure my car's WHP?
A chassis dyno pull at a performance shop, typically $75 to $150 for a few pulls. Do a baseline before modifications and re-test after on the same dyno; the gain is the truth, the absolute number is trivia.
Does a tune increase WHP or crank HP?
Both, proportionally: a tune that adds 10% at the crank adds about 10% at the wheels, since the drivetrain loss percentage stays roughly constant. Tuners quote crank gains for marketing; the dyno sheet shows wheel gains for honesty.
Why does my car feel slower than its HP rating?
Weight, gearing, and where the power lives in the rev range matter more than the peak number. A 300-hp car at 3,800 lbs with tall gearing can feel lazier than a 250-hp car at 3,000 lbs with short gearing. WHP per pound and the area under the torque curve are the real story.