This is a question we get asked often. You want to buy a fifth wheel, but don't want to buy a new truck, or you have your eye on a truck and want to know if it can handle the RV you want. Totally get it, so let's take a look at how to figure this out!
With any truck, there are two numbers to be concerned with for towing: tow capacity and cargo capacity. Actually, three numbers, but we'll get to that.
This number is based around the engine and transmission. It's how much your truck can pull (and stop) safely. This number can be found in towing charts like this: www.fleet.ford.com…
To find yours, simple google “towing charts for YEAR and MODEL”.
You'll find your model and a grid, showing the truck style, engine, and axle ratio. Line these up and there's your tow capacity. Pretty simple. The reason these numbers are published in a chart like this for each model year, is they are the same number regardless of what options (besides engine and gear ratio) are installed on the truck.
For this exercise, we'll look at a 2017 F350 Super Duty (6.7L Power Stroke) SRW (Single Rear Wheel) 4×4 Crew Cab 8′ Box with a 3.55 Axle Ratio. From the link above, we find this:
The number highlighted to the right is the towing capacity, but if you see the (4)(5) and check the notes, you'll see that 18,000 is the actual capacity for a fifth wheel and 20,000 is for a gooseneck.
Note: There will be separate charts for fifth-wheel and conventional (bumper pull), so be sure to use the correct chart.
In this chart, are two numbers: towing capacity (on the right), and GCWR (Gross Combined Weight Rating), which we'll touch on later. The max is 18,000lbs for fifth-wheel towing. Many people, including many truck dealers, stop here and think this is what this truck can tow. However, this is only half of the picture. Of equal importance is cargo capacity.
A truck's cargo capacity is how much weight it can actually hold in (or on) the truck itself. This number is calculated using the truck's GVWR (Gross Vehicle Weight Rating) minus the truck's curb weight when it left the factory. Since every truck coming off the factory line will be different based on installed options, the cargo capacity is specific to each truck. This number will be found on a yellow “Tire and Loading Information” sticker in the driver's side door jamb and looks like this:
The sticker above is also from a 2017 F350 Super Duty SRW (Single Rear Wheel) 4×4 Crew Cab 8′ Box with 3.55 Axle Ratio. Notice that it says “the combined weight of occupants and cargo”. This means you, passengers, pets, tools, and even the fifth wheel hitch itself (the part that mounts to the bed of the truck). Those items alone will usually account for a few hundred pounds, considering most hitches weigh about 100lbs alone.
Let's just say that it's just a 200 lb. driver and a 100 lb. hitch with nothing else. In the example above, that leaves 3255 lbs. (3555 – 300) of true cargo capacity.
A fifth-wheel will have between 20% and 25% of its total weight on the pin. That means 20-25% sitting in the bed of your truck. The percentage will vary depending on how things are loaded, if there's a generator in the front bay, etc. But, it gives us a range to work with. Knowing this, it's simple math to calculate a general range of the largest fifth-wheel by multiplying your true cargo capacity by 4 for the low end (25% on the pin) and 5 for the high end (20% on the pin).
With our example of 3255 lbs., we come up with 13,020 – 16,275. That is the maximum size of the fifth wheel. Notice, the high-end number is a LOT smaller than the 18,000 towing capacity! This is why cargo capacity is so important and why most large fifth wheels need a DRW (Dual Rear Wheel / Dually) truck.
Gross Combined Weight Rating
If you are under your tow rating and under your cargo capacity, you generally don't need to worry about GCWR, but you should know what it is. GCWR is simply the maximum combined weight of the truck and everything connected to it or in it.
What if I add Air Bags?
While air bags can help level out an over-loaded truck by jacking up the rear, it changes nothing about the truck's cargo rating. The truck will be level, but still over loaded.
What about RAM and Chevy?
You'll find that all of the big 3 truck makers (Ford, Dodge, Chevy) all sell equally capable trucks. Pick the brand you like with the features you like, along with the right specifications for towing and you're golden! Just be aware that RAM does make some strange combinations. When we were truck shopping, we were shown a RAM Cummins Dually with a 68RFE transmission and 3.73 axle ratio that had a great payload (cargo) capacity, but a tow rating of 18,660. Of course, the dealer insisted it could tow a 20K fiver because it's a Dually after all. ? Know your numbers, because there's a good chance the dealer will not.
You'll see a lot of arguing online about this topic, with references to the “tow police” whenever someone with an over-loaded truck want's to justify pulling a 20K toy hauler with a single rear wheel truck. But, as you can see, it's really just very simple math. That said, I've seen some pretty impressive numbers on the 2020 Ford SRW trucks that put it into the 18K range for fiver capacity. But, know your numbers. You are the one responsible for your family's safety on the road. Be safe!