Originally Posted by Jim Flower
The specifications have become irrelevant, which is why no insurance company in the US has reneged on paying a claim due to a vehicle towing a trailer that has exceeded the manufacturer's "tow rating".
But if you are sued after an accident with your trailer and the plaintiff's lawyer finds out you had something overweight, you could be in trouble.
Here's a link to definitions about all these acronyms and terms:
Tow Vehicle Sizing
I'm not sure I agree with everything there though. For ex., the idea that you should not exceed 80% of the any limit. That's conventional wisdom and I've never seen proof of it. While it can be argued that the closer you get to a maximum rating of anything, the more risk you have, that is not necessarily so. Some of it may come from the belief that all the manufacturers lie about their truck ratings so they can sell more vehicles. That hasn't been proven either.
As I understand it, calculating tongue weight against payload or other weight maximums is not done by taking the total tongue weight of an unhitched trailer. The WD hitch transfers weight both ways—some to the trailer axle(s) and more to the tow vehicle axles. While there is a formula for this, most of us wouldn't understand it, so roughly it is 1/3 to the trailer axles and 2/3 to the truck axles. I see frequent posts on the Forum that don't take this into account when computing proper weights for the tow vehicle.
And last, tongue weight as stated by the manufacturers cannot be trusted. I have seen Airstream give different weights in different publications for the same model in the same year. And, filled propane tanks, heavier than OEM batteries, spare tire and other options can add a lot to tongue weight.