I know it’s complex; GVM, GCM, ATM and more. But, limits are limits, and here’s one that’s overlooked.
The axle limits.
Each vehicle has a maximum load the front and rear axle can take, and it’s often pretty low. And the thing about all these limits is that the first limit you hit is what dictates your towing capacity; if you hit GCM then you can’t add any weight to the vehicle even if the GVM limit isn’t reached.
The problem with the rear axle load limit is odd but important. Every kilogram of weight you add to the towball translates to more than a kilogram of weight on the rear axle, and proportionally less on the front axle. And, all trailers change their towball mass depending on the height of the towball, but tandem axle trailers change opposite to single axle trailers.
All this is explained and demonstrated in the video below, and don’t forget the Tow Weights Calculator: