Originally Posted by storm
The rule of thumb I have always used in towing is 1ft. lb. of torgue will tow 30 lb. of weight.
That's a very simple method to estimate tow rating. But tow rating tells you only how much weight you can pull
without burning up something in the drivetrain. It doesn't tell you how much hitch weight you can haul
without overloading the suspension of your tow vehicle.
Your formula says my '99.5 PowerStroke with 500 lb/ft torque could tow a trailer that grossed 15,000 pounds. Ford's fifth wheel tow rating was about 13,000 pounds, and that was severely overstated. In the real world, my 5er that grossed less than 8,000 pounds overloaded that tow vehicle.
So I would conclude that your formula, and manufacturers' tow ratings, are not very useful. You must also compare available unused payload capacity to the wet and loaded hitch weight of the trailer. That requires weighing the rig to determine actual wet and loaded weights.
The problem is that the tow rating, and your formula that estimates tow rating, ignores hitch weight. Most tow vehicles with single rear wheels can pull
trailers that get close to the weight of the tow rating, but they cannot haul
the hitch weight of a tandem-axle trailer that weighs anywhere close to that much.
When dealing with wagon-style farm trailers with no hitch weight, such as cotton trailers and grain trailers, your formula gives a decent estimate of the weight of the trailer your tow vehicle can move out of the field and onto the road then to the gin or elevator a few miles up the road. But not for a fifth-wheel RV trailer with 18% hitch weight towed with an SRW pickup.