How to use this calculator
- Enter road speed. Set the steady target speed.
- Enter aero values. Set drag coefficient, frontal area and air density.
- Enter rolling load. Set rolling resistance coefficient and vehicle mass.
- Read power. Review road-load force and wheel power.
How it works
Aerodynamic drag is 0.5 x density x Cd x area x speed^2. Rolling force is Crr x mass x g. Power is total force multiplied by speed.
Use this with the vehicle speed calculator and wheel torque calculator when checking whether gearing can hold a target speed.
Worked example
Verified against the live calculator
At 70 mph, a 1500 kg vehicle with Cd 0.32, 2.2 m2 frontal area and Crr 0.012 needs roughly 19 kW at the wheels for level-road drag plus rolling resistance.
Frequently asked questions
What is road-load horsepower?
It is the wheel power needed to overcome aerodynamic drag and rolling resistance at a steady speed on level ground.
Does this include drivetrain loss?
No. The result is wheel power. Divide by drivetrain efficiency to estimate engine power.
Why does speed matter so much?
Drag force rises with speed squared, and drag power rises roughly with speed cubed.
What should I use for rolling resistance?
Passenger-car tires are often near 0.010-0.015, but tire type, pressure, load and surface can move it.
Method & assumptions
- Assumes steady speed on level ground with no wind and no drivetrain loss.
- Grade, acceleration, accessory loads, tire pressure, yaw, ride height and cooling drag are not included.