How it works
To hold a steady speed, the engine must overcome three forces: gravity on the grade, tire rolling resistance and air drag.
Power at the wheels = v × m × g × (sin θ + Crr × cos θ) + ½ × ρ × CdA × v³
Engine power = wheel power ÷ drivetrain efficiency
v is speed, m mass, g gravity, θ the angle of the grade, Crr = 0.012 the rolling resistance, ρ = 1.225 kg/m³ sea-level air density, and CdA the combined drag area of truck and trailer (from 22 ft² for a low trailer to 50 ft² for a tall camper). Drivetrain efficiency is taken as 85%.
Worked example
A 15,000 lb truck and travel trailer climbing a 6% grade at 55 mph.
- Grade: about 132 hp at the wheels; rolling resistance about 26 hp; air drag about 57 hp.
- Total at the wheels ≈ 215 hp, so about 253 hp at the engine.
- With a 400 hp engine that is 63% of rated power: workable, and it could hold about 73 mph if it had to.
Engine horsepower needed at 55 mph
Tall trailer, sea level.
| Grade | 10,000 lb | 15,000 lb | 20,000 lb |
|---|---|---|---|
| 0% | 87 hp | 98 hp | 108 hp |
| 2% | 122 hp | 150 hp | 177 hp |
| 4% | 156 hp | 201 hp | 246 hp |
| 6% | 191 hp | 253 hp | 315 hp |
| 8% | 225 hp | 304 hp | 383 hp |
Getting up the hill
On long climbs, power is only half the story; heat is the other. Watch the transmission and coolant temperatures, use tow/haul mode, and let the speed drop rather than running at full throttle for miles. Downhill, the same physics works in reverse: use engine braking so the brakes stay cool (see the stopping distance calculator).
Gearing decides whether the engine is in its power band at that speed; check it with the engine RPM calculator. On flat ground, drag dominates at highway speed: the trailer drag calculatorshows how much slowing down saves.
Common mistakes
- Using brochure horsepower as continuous power. Engines can’t run at peak for long climbs.
- Forgetting the truck’s own weight. Use the combined weight of everything that is moving.
- Ignoring altitude. Non-turbo engines lose noticeable power on high passes.
- Staying in overdrive. Low RPM means low power; downshift before the engine bogs.