What drag is
Anything moving through air pushes it aside, and the air pushes back. This aerodynamic drag is negligible at walking pace, but it grows with the square of speed and becomes the main force a car fights on the highway and that a cyclist fights above about 10 mph.
The drag equation
F = ½ × ρ × v² × Cd × A
- ρ: air density, 1.225 kg/m³ at sea level and 59 °F
- v: speed relative to the air (add a headwind, subtract a tailwind)
- Cd: drag coefficient, which depends on shape
- A: frontal area
The power needed to overcome drag is P = F × v, so it grows with the cube of speed. The calculator also gives the energy spent against drag per 100 miles, which is what drives highway fuel economy.
Worked example: a sedan at 70 mph
- Speed 70 mph (31.3 m/s)
- Cd = 0.30
- Frontal area 23.7 sq ft (2.2 m²)
F = 0.5 × 1.225 × 31.3² × 0.30 × 2.2 ≈ 396 N (89 lbf). Power = 396 × 31.3 ≈ 12.4 kW, or 16.6 hp, just to push through the air. Over 100 miles that is about 17.7 kWh at the wheels, before rolling resistance and drivetrain losses.
Why slowing down saves so much
| Speed | Relative drag | Relative power |
|---|---|---|
| 55 mph | 62% | 49% |
| 65 mph | 86% | 80% |
| 70 mph | 100% | 100% |
| 80 mph | 131% | 149% |
Going from 70 to 80 mph needs about half again as much power against drag. For an electric car, that is a direct hit on range.
Typical drag coefficients
| Object | Cd |
|---|---|
| Streamlined modern EV | 0.20–0.24 |
| Sedan | 0.25–0.30 |
| SUV or pickup | 0.35–0.45 |
| Semi truck | 0.6–0.9 |
| Cyclist, upright | 0.9–1.0 |
| Sphere | 0.47 |
| Flat plate facing the flow | 1.17–1.28 |
Engineers often quote CdA (drag area) instead, because a small car with a mediocre Cd can still beat a large one with a good Cd.
Limits
The equation assumes Cd is constant, which holds for cars and cyclists over everyday speeds but not near the speed of sound or for very small, slow objects. Air density falls with altitude and heat, which reduces drag; enter a lower value for mountain roads or hot days.