How it works
Air drag is the force needed to push the trailer’s frontal area through the air. It depends on shape, size, air density and speed.
Drag force F = ½ × ρ × Cd × A × v²
Drag power P = F × v
ρ = 1.225 kg/m³ is sea-level air density, Cd the drag coefficient (shape), A the frontal area and v the air speed. Because power grows with v³, a small cut in speed makes a big difference.
Worked example
An 8 ft wide, 10 ft tall travel trailer with a rounded front (Cd 0.65) at 65 mph.
- Frontal area: 80 sq ft (7.4 m²); speed 29.1 m/s
- Drag force: ½ × 1.225 × 0.65 × 7.4 × 29.1² ≈ 2,500 N = 562 lbf
- Power: 2,500 × 29.1 ≈ 73 kW = 97 hp; at 55 mph it drops to about 59 hp.
Drag by speed
8 × 10 ft travel trailer with a rounded front.
| Speed | Drag force | Power to overcome it |
|---|---|---|
| 45 mph | 269 lbf | 32 hp |
| 55 mph | 402 lbf | 59 hp |
| 60 mph | 479 lbf | 77 hp |
| 65 mph | 562 lbf | 97 hp |
| 70 mph | 651 lbf | 122 hp |
| 75 mph | 748 lbf | 150 hp |
Drag coefficients used
| Trailer front | Cd (approx.) |
|---|---|
| Flat front: cargo trailer, toy hauler | 0.8 |
| Rounded front: most travel trailers | 0.65 |
| V-nose cargo trailer | 0.6 |
| Low and smooth: teardrop, pop-up closed | 0.45 |
| Open: boat, utility, car hauler (loaded) | 0.55 |
These are rough effective values for planning, not wind-tunnel results for a specific model. The effect of speed is the same whatever value you use.
Cutting drag
Speed is the biggest lever: every 5 mph slower cuts drag power by roughly 20%. After that, shape and size matter: a V-nose or rounded front, lowered roof items and a smaller gap between truck and trailer all help. For the effect on a whole trip, see thetowing fuel cost calculator, and for drag on a climb, thetowing horsepower calculator.
Common mistakes
- Using total height from the ground. Measure the body, not the space below it.
- Ignoring wind. A headwind adds directly to the air speed.
- Assuming weight is the main fuel cost. On flat highways, drag usually costs more.
- Treating the result as exact. Real drag depends on details this formula can’t see.