How Much Horsepower Do You Need to Tow?
Flat roads take surprisingly little power. Hills are what decide it, and the amount follows directly from weight, grade and the speed you want to hold.
Last updated · 8 min read
Ask how much horsepower you need to tow and you will hear everything from "torque is all that matters" to a fixed number per thousand pounds. The physics is more useful than either. Moving a trailer on level ground takes very little power. Climbing a long grade at highway speed takes a lot, and you can calculate how much.
The Formula for Climbing a Hill
Climbing a grade means lifting the whole rig. Lifting weight at a given rate takes power, and in US units the arithmetic works out neatly:
Rolling resistance adds about weight × 0.01 × mph ÷ 375 for highway tires
The grade term is exact physics: it is the power needed to raise the rig's weight at the rate the road climbs. The rolling-resistance term uses a typical coefficient of about 0.01 for tires on pavement. Aerodynamic drag comes on top of both. It grows with the cube of speed, so at highway speeds a tall, square travel trailer can need as much power to push through the air as it does to climb a moderate grade.
Horsepower Needed by Weight, Grade and Speed
Wheel horsepower for grade plus rolling resistance, without aerodynamic drag. Divide by about 0.85 for a rough engine figure.
| Combined weight | 6% grade, 40 mph | 6% grade, 55 mph | Level, 65 mph (rolling only) |
|---|---|---|---|
| 10,000 lb | 75 hp | 103 hp | 17 hp |
| 15,000 lb | 112 hp | 154 hp | 26 hp |
| 20,000 lb | 149 hp | 205 hp | 35 hp |
The level-road column shows why a modest engine can feel fine towing on the flat and then struggle on the first long hill. Rolling resistance at 65 mph takes only a few dozen horsepower even at 20,000 lb. The grade multiplies the requirement several times over.
How Tow Ratings Are Tested: SAE J2807
US truck makers rate towing capacity to SAE J2807, a standard first published in 2008. Rather than a horsepower rule, it sets performance tests the fully loaded truck and trailer must pass at the claimed weight:
- Davis Dam grade. An 11 mile climb of 3,500 feet on Arizona Route 68, an average grade of about 6%, starting in 100 °F heat with the air conditioning on full. Single-rear-wheel trucks must hold at least 40 mph and dual-rear-wheel trucks at least 35 mph.
- Acceleration on level ground. 0 to 30 mph in 12 seconds and 0 to 60 mph in 30 seconds for single-rear-wheel trucks, with a 40 to 60 mph passing test in 18 seconds.
- Hill starts and parking. Starting five times on a 12% grade, and holding the rig there on the parking brake alone.
- Braking, handling and sway tests at set speeds and distances.
The Davis Dam figures line up with the formula above. A truck that holds 40 mph up a 6% grade at its rated combined weight has to deliver the wheel horsepower in the first column of the table, plus drag, while coping with the heat. That is a stronger test of real towing ability than any single horsepower or torque number.
How Many Pounds Can One Horsepower Pull?
On level ground, almost any amount, slowly. Horsepower is a rate of work, so a small engine geared low enough could crawl with a very heavy load. The useful question is how much weight one horsepower can carry up a grade at a given speed. Rearranging the formula, one horsepower at 40 mph on a 6% grade lifts about 375 ÷ (0.06 × 40) = 156 lb of combined weight. That is why a heavy rig needs so much more power to hold speed on hills than its flat-road behaviour suggests.
Torque vs Horsepower for Towing
Torque gets the credit for towing, and it matters, but not in the way it is usually described. The force that pulls the trailer is torque at the wheels, which is engine torque multiplied by the gear ratios. A strong low-RPM torque curve lets a truck pull hard without revving and downshifting, which is why diesels are popular for towing. But speed up a hill is set by power, and power is torque times RPM. Two trucks with the same horsepower can climb a grade at the same speed, one at low RPM with lots of torque and one at higher RPM in a lower gear.
In practice, look for an engine that makes plenty of power across a broad RPM range, gearing that keeps it there, and adequate cooling. For the difference between the two measurements, see horsepower vs torque, and for wheel torque the torque to weight calculator.
The Weight Ratings That Matter More
Horsepower rarely sets the legal or safe limit. The truck's ratings do:
- Towing capacity: the maximum trailer weight, as tested under J2807.
- GCWR (gross combined weight rating): the maximum for truck, passengers, cargo and trailer together.
- Payload: what the truck can carry, including passengers and the trailer's tongue weight.
- Tongue weight: GM owner's manuals specify 10 to 15% of the loaded trailer weight for conventional trailers and 15 to 25% for fifth-wheel and gooseneck trailers.
Many owners also follow a common rule of thumb of keeping the trailer to about 80% of the rated capacity, to leave a margin for heat, hills and wind. It is a guideline, not part of the rating.
Altitude and Heat
Naturally aspirated engines lose power as the air thins, roughly 3% per 1,000 feet, which is exactly where long mountain grades tend to be. Turbocharged engines hold their power much better at altitude. Use the altitude horsepower loss calculator to see what your engine will have at the top of the pass, and the wheel to crank horsepower converter to compare dyno and brochure figures.
Frequently Asked Questions
It depends on the hills. A 10,000 lb combined weight needs about 17 wheel hp to roll at 65 mph on the level, but about 75 to hold 40 mph up a 6% grade, before aerodynamic drag and drivetrain losses.
Both matter. Low-RPM torque, multiplied by gearing, makes pulling away and pulling at low speed easy. Horsepower sets how fast you can climb a grade. A good tow vehicle makes strong power across a wide RPM range.
On level ground a lot, slowly. Up a 6% grade at 40 mph, one wheel horsepower lifts about 156 lb of combined weight, using HP = weight × grade × mph ÷ 375.
US truck makers use SAE J2807, which includes the Davis Dam climb, an 11 mile, roughly 6% grade in 100 °F heat where single-rear-wheel trucks must hold 40 mph, plus acceleration, braking and handling tests.
A common guideline to keep the trailer at or below about 80% of the truck's rated towing capacity, leaving a margin for heat, grades and wind. It is not part of the official rating.
Not necessarily. Diesels make strong torque at low RPM and are efficient under load, which suits heavy towing. A gasoline engine with enough power and the right gearing can tow the same weight, usually at higher RPM.