How this calculator works
Power leaves the engine and passes through the gearbox, gear meshes, bearings, driveshaft, universal joints or constant velocity joints and the final drive before it reaches the tires. Each component loses a small share, and the shares multiply rather than add. The total efficiency is therefore the product of the individual efficiencies, raised to the count where several identical parts are in series. Wheel horsepower is engine power times that total. Rear-wheel-drive cars carry a driveshaft and universal joints that front-wheel-drive cars replace with transaxle gearing and constant velocity joints, so the default chains differ.
Formula
Total efficiency = gearbox x mesh^meshes x bearing^bearings x final drive x (driveshaft x universal^joints for RWD, or CV joint^joints for FWD). Wheel power = engine power x efficiency. Engine power = wheel power / efficiency. Loss = engine power - wheel power.
Worked example
A 400 hp rear-wheel-drive car with a 95 percent gearbox, two 98 percent gear meshes, two 99 percent bearings, a 93 percent final drive, a 99 percent driveshaft and one 99 percent universal joint has a total efficiency of 0.95 x 0.98^2 x 0.99^2 x 0.93 x 0.99 x 0.99 = 0.8151. Wheel horsepower is 326.0 hp and the loss is 74.0 hp.
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Frequently asked questions
Is drivetrain loss a fixed percentage?
No. It varies with load, speed, oil temperature and gear selected. The percentage is usually worse at light load and in lower gears, so a single efficiency is an average for a full-throttle dyno pull.
Why is the loss higher in some dyno comparisons?
Chassis dyno operators sometimes apply a rule of thumb for a particular drivetrain, and tire slip, tie-down strap losses and dyno calibration all affect the reading. A build sheet with measured engine power is a better reference than a guessed percentage.
Does an automatic transmission lose more than a manual?
Often yes, especially a torque converter without lock-up. A modern locked converter and a dual-clutch transmission can be closer to a manual. Set the gearbox efficiency to the value you believe for your transmission.
Should I include tire losses?
The final drive efficiency can include the tire-to-roller loss if you are comparing with a chassis dyno. For a pure drivetrain calculation, leave tire slip out and compare wheel power at the hubs.