Second this most heartily. All of the tires on my motor home looked just fine and the vehicle had very low mileage. I started out on a trip one summer and immediately noticed some vibration in the front end. Got about 50 miles down the road and limped back to the city. I went to my Ford dealer and he found broken belts in two of the tires. That was when I learned that, because recreational vehicles sit for long periods of time and undergo a lot of stress from loading, five years is about the outside limit for tires. It’s been five years since that happened and I have a new set of Michelins sitting in the garage waiting out the corona virus to be installed.
Ok, so if I have this straight:
(curb weight of the truck) + (dry weight of the trailer) + (passengers, water, fuel, all cargo between the truck AND the trailer) cannot exceed the GCWR.
So, if I assign 2000 lbs to the passengers, etc. variable, I can rewrite this formula to be:
GCWR - (curb weight of truck) - 2000 = max dry weight of trailer
Does that sound right?
So, for example, Carmax is listing a 2019 F-150 XLT, 4-door crew cab, 5.0L engine. In Ford’s towing guide, the smallest GCWR listed for this particular configuration is 13,100 lbs. According to Carmax, the curb weight is 4580 lbs.
13,100 - 4580 - 2000 = 6520 max dry trailer weight
The heaviest trailer on our list is 4400 lbs dry weight. Seems like I’m good to go with this truck, but I’m still not quite understanding how the GVWR figures into this. If the GVWR is for the truck + people, fuel, cargo, etc. only, shouldn’t it already be accounted for in the GCWR?
Not that I would discourage you from grappling with the math to learn it on your own terms, but have you tried an online calculator?
If nothing else, it’s an independent double check on your work.
Have the dealer put the details of the trailer into the purchase agreement. Now you’ve got a warranty for fitness for a particular purpose.
I think you’re on the right track, but the “curb weight” given by manufacturers is universally too low. IIRC, they weigh a basic, low-end model with a regular cab and assign that as the curb weight of that class of vehicles. Notice in my post that I had a very similar truck, but with a shorter bed (6’ 4") and a shorter cab (quad, instead of crew). And still was 1200 lbs heavier than the weight you’re using. It seems unlikely the truck brands are so different that similar vehicles will have 1200 pounds of weight difference.
I would lurk around some RV-ing web forums and try to find someone with a similar truck who can give an approximation of your (proposed) truck’s weight.
I’d calculate it like this:
13,100 (GCWR) - 5800 (Guess of actual truck wt. with pax) = 7300. This would be your max trailer weight. Subtracting a safe 15% of that (for your personal effects/options) gives you a ballpark of 6200 dry for your trailer. Since the trailer you’re considering is 4400 dry, you’re well within the 80% load factor you mentioned at the beginning.
Again, I can’t imagine you’d have any trouble with a modern half-ton + tow package handling a 20 foot trailer. The new trucks have astounding power for their size, and you’re well within all of the numbers. If you find you’re a teensy bit over the GVWR, check the individual axle ratings (they’ll be listed inside the door jamb). You’ll probably find you’re not even close to maxing out those, so nothing’s gonna break.
Again, hoping I don’t sound like a broken record; But a really good WD hitch will make all the difference. It makes the whole rig act (sort of) like a single unit, instead of two large vehicles with a hinge in between.