How to Calculate Towing Capacity

Before hooking up a camper, a boat or a loaded trailer, it pays to know how much the truck can really pull. The number in the brochure is a starting point; the load inside the cab and bed changes it. Here is how to work out the figure for your own setup.

What towing capacity means

Towing capacity is the most weight a vehicle can pull safely. Staying under it protects the truck, the trailer and everyone on the road, and it keeps you clear of fines for overloading.

The factory figure is general. What you can actually tow depends on a few things:

• How the vehicle is built: the strength of the frame, the engine and the type of transmission.
• What is already on board: passengers, cargo and gear in the truck.
• How the weight is spread across the vehicle and the trailer.

When in doubt, aim lower. An overloaded truck strains its engine, transmission and brakes, and a breakdown with a trailer attached is no fun.

The terms on the spec sheet

• Gross Vehicle Weight Rating (GVWR): the most the vehicle may weigh standing still, itself plus passengers and cargo. Ratings run from Class 1 (6,000 lbs) to Class 8 (33,001 to 80,000 lbs).
• Gross Trailer Weight (GTW): the trailer plus everything loaded on it.
• Gross Combined Vehicle Weight Rating (GCVWR): the limit for truck and trailer together, with all cargo and passengers.
• Gross Axle Weight Rating (GAWR): the most each axle can carry.
• Tongue weight: the downward push of the trailer on the hitch.
• Curb weight: the vehicle with a full tank and all fluids, but no people or cargo.
• Payload: what the vehicle can carry in the cab and bed, not counting its own weight.

Three steps to the number

1. Look up the GCVWR

It is in the owner's manual. Truck, payload, tongue weight and trailer together must stay below it.

2. Find the curb weight

Also in the owner's manual, or on the sticker in the driver's door frame.

3. Subtract

Towing capacity equals the GCVWR minus the curb weight. A truck with a 25,000-lb GCVWR and an 8,000-lb curb weight can tow 17,000 lbs.

Payload is a different limit

Towing capacity is what the truck can pull; payload is what it can carry. Payload equals the GVWR minus the curb weight, and it covers passengers, cargo in the bed and any aftermarket add-ons. Towing loads the engine and transmission, while payload loads the suspension and brakes, so both numbers matter.

A worked example

Take a heavy-duty pickup with a 25,000-lb GCVWR and an 8,000-lb curb weight. On board are a 200-lb driver, a 180-lb passenger and 300 lbs of tools in the bed, a payload of 680 lbs. Available towing capacity is the GCVWR minus curb weight and payload, so 25,000 - (8,000 + 680) = 16,320 lbs.

Then leave a margin. A common rule is to stay at or under 90% of the calculated figure, which allows for road conditions and loads that weigh more than expected. Here that means 16,320 × 0.9 = 14,688 lbs.

The equipment that makes it work

The truck is only part of the chain. A trailer rated for 5,000 lbs does not help if the vehicle tops out at 3,500, and every piece in between has its own rating.

• Hitch receiver: sold in Class I to V for different loads; heavy hauling usually calls for Class IV or V.
• Ball mount: standard or adjustable. Common sizes are 1¼, 2, 2½ and 3 inches, and an adjustable mount helps with trailers of different heights.
• Pin and clip: hold the receiver and ball mount together; pick sturdy, corrosion-resistant ones.
• Hitch ball: has to match the trailer's coupler.
• Wiring harness: the law requires working trailer lights, so make sure it fits both truck and trailer.
• Weight distribution hitch: spreads the load between the axles of the truck and trailer for a smoother ride, especially on long trips.

Check all of it for wear regularly, and replace a worn part even before its rated life is up.