This thread sent me down a mental tangent. Is the GVWR actually a legal limit in Ontario? If so, where does it appear in the HTA?
No, and most passenger cars do not carry a declared GVWR, only pickups and larger, nor can the registered gross weight of a passenger (blue) plated car be changed, only commercial (black) plated pickup trucks or larger.
Gross REGISTERED weight (GRVW, not to be confused with manufacturer placarded GVWR or Gross Vehicle Weight Rating) CAN be changed simply by visiting an MTO office and paying extra for your plate sticker. My old pickup truck (since sold) was registered for 30,000# as that's about what I weighed with the truck and horse trailer with 4 draft horses in it.
What is NOT negotiable so far as the letter of the law is axle (GAWR), tire, and rim ratings, or the GVWR in the case of pickup trucks. You can register a pickup for 1/2 ton grocery getter pickup truck for 100,000# if you wanted, but as soon as you overload either the axle, the rims, or the tires beyond their stated capacities (or exceeding the GVWR as a whole), you're no longer legal. The same goes for a car so far as axle, rims, and tires (since GVWR is often not stated), so the number of passengers, junk in the trunk, and tongue weight on a trailer matters a lot.
If the trailer has brakes, as well, they MUST be functional, otherwise again, not legal.
In order for an insurance company to deny a claim based on overloading a vehicle beyond it's tow limits they'd have to be able to demonstrate that you were grossly negligent - IE, trying to tow a 30' 13,000# travel trailer with the 300, or towing a trailer that grossly overloaded the axle GAWR to the point of catastrophic failure. Towing a 2500# trailer on a vehicle rated in NA for 1000 (or more depending on configuration, it can vary a lot) would be very difficult for then to prove that, especially when any lawyer worth his salt would quickly introduce into evidence that in Europe many of the identical vehicles are rated to tow 3 to 5 times as much weight as they are rated for here in NA - some 300's over there are rated in the 3000#+ capacity range for example. Those differences are mainly due to poorer driver training in NA vs Europe, and more difficult and challenging terrain here vs there.