Calculators · Vehicle
Aerodynamic drag, rolling resistance and top speed calculator
At motorway speeds most of a car’s power goes into pushing air aside. Drag rises with the square of speed and the power to overcome it with the cube, which is why a little more top speed needs a lot more power. These calculators give aerodynamic drag and rolling resistance at any speed, estimate top speed from power, and estimate frontal area when the manufacturer does not publish it.
Aerodynamic drag force and power
Calculates the air resistance on a vehicle and the power needed to overcome it at a chosen speed, from its drag coefficient and frontal area. Doubling speed quadruples the force and multiplies the power by eight.
F = ½ × ρ × Cd × A × v² P = F × v
Rolling resistance
Tyres flex as they roll, which costs energy. Rolling resistance is almost constant with speed, so it dominates in town while drag dominates on the motorway. Low-rolling-resistance tyres cut it noticeably.
F = m × g × Crr P = F × v
Top speed from power
Finds the speed at which the power reaching the wheels exactly balances drag and rolling resistance. It assumes the gearing lets the engine reach peak power at that speed; many cars are limited by gearing or an electronic limiter first.
P × η = ½ρ·Cd·A·v³ + m·g·Crr·v (solved for v)
Frontal area estimate
When the frontal area is not published, it can be estimated from the vehicle’s width and height. The body is not a rectangle, so a shape factor of about 0.85 for cars and 0.9 or more for vans and trucks is applied.
A ≈ width × height × shape factor
Worked example: a 150 kW saloon
- Cd 0.30 and frontal area 2.2 m² give a CdA of 0.66 m².
- At 100 km/h drag is ½ × 1.204 × 0.66 × 27.8² = 307 N, needing 8.5 kW.
- Rolling resistance for 1,500 kg at Crr 0.010 adds 147 N, or 4.1 kW.
- With 127.5 kW at the wheels, drag and rolling resistance balance at about 240 km/h.
Typical drag coefficients
| Vehicle | Cd | Typical CdA |
|---|---|---|
| Streamlined electric saloon | about 0.20 – 0.24 | about 0.45 – 0.55 m² |
| Modern family car | about 0.26 – 0.32 | about 0.55 – 0.70 m² |
| SUV | about 0.33 – 0.40 | about 0.85 – 1.10 m² |
| Pickup truck | about 0.40 – 0.50 | about 1.1 – 1.4 m² |
| Heavy truck | about 0.55 – 0.70 | about 5 – 7 m² |
Related
About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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