I am quoting an article by Shane Zepplin of Towmaster here. I think this does a pretty good job of breaking it down. I hope that it helps.
John was on his way to the jobsite. He had just purchased a new skid-steer loader, and this was going to be his first job with the new equipment. The research he did online helped him determine that this particular model would allow him to finish a job 25 percent faster than his old model. This meant he could earn more money, which justified the purchase of the skid-steer loader and the new trailer. Little did he know that what he planned to save in efficiency, he spent on the Department of Transportation (DOT) ticket for being overweight on his gross vehicle weight rating (GVWR).
This is a common occurrence these days as more states are looking for revenue to subsidize their budgets. GVWR laws are not new—authorities have just been lax about enforcing them in the past. They are now enforcing the laws, not only for revenue but for safety reasons and to prevent wear-and-tear on the highways, which also saves taxpayer’s money.
John received his ticket because he did not understand GVWR, which is the case for many contractors. Even manufacturers have different interpretations of GVWR because the government leaves it up to each manufacturer to rate its trailers as they see fit. The bottom line: Whatever the tag on the trailer says for GVWR better be higher than what the scale reads.
John’s new skid loader with the attachment weighed 9,700 pounds, so he thought his trailer, with a 9,990-pound GVWR, was sufficient. However, he did not consider the trailer’s weight (or any attachments on his skid loader). Even though his state recognizes tongue transfer, the most he could transfer to his tow vehicle was 1,200 pounds. With the trailer weighing in at 2,600 pounds, his total weight was 11,100. He was over by 1,110 pounds.
John cannot be blamed 100 percent because his dealer should have explained GVWR and asked John some pertinent questions about his tow vehicle. The dealer also should have known the weight of the skid-steer loader and attachment.
Trailer GVWR is the amount of weight the vehicle can carry. Some manufacturers rate the GVWR at the lowest component rating, while others include the trailer’s weight on top of the trailer capacity. If you have a trailer that weighs 2,600 pounds, that weight must be included in the capacity formula to stay under the GVWR. You also must consider the addition of accessories, such as pallet fork holders, a heavier parking jack, winch and battery and other items. Plus, you have to add the weight of any attachments you haul along with your skid loader. Even adding a planer or grapple bucket can put you over the limit.
Manufacturers have leeway in determining the GVWR of their trailers. One method is by taking the lowest component rating and calling that the GVWR. In most cases, that would be the axle or tire rating. In other cases, that would be the hub and wheel rating. Either way, the GVWR is determined at the axle. With this method, if your axle rating is 6,000 pounds per axle, and you have two axles, your GVWR would be 12,000 pounds.
Other manufacturers take that rating and add the trailer’s weight into the GVWR. If the capacity is rated at 12,000 pounds (determined by the axle capacities), and the trailer weighs 2,600 pounds, they would rate their trailer at 14,600 GVWR.
In the first case, the capacity rating of that trailer is determined by subtracting the weight of the trailer from the GVWR of 12,000 pounds. If the trailer weighs 2,600 pounds, then the payload capacity of that model is 9,400 pounds.
In the second case, the trailer’s weight is already in the GVWR, so the payload capacity of that trailer is 12,000 pounds. This sounds simple enough, but it actually is not. While the GVWR is rated at 14,600 pounds, the GAWR or Gross Axle Weight Rating is still only 12,000 pounds and you cannot exceed that capacity rating. In this case, you still have to include the 2,600-pound trailer weight. That weight must be transferred to the tow vehicle, but that transfer is also limited by the hitch capacity. If the hitch is only rated at 1,200 pounds, you cannot transfer the entire weight of the trailer to the tow vehicle. This reduces your payload capacity by the difference of 1,400 pounds, so the actual capacity would be 10,600 pounds. This is still higher than the 9,400 pounds in the first example but can throw you off when you are calculating the payload capacity.
This can be confusing to both dealers and consumers and, quite frankly, to DOT officers and even some state legislators. Like any law, it is open to interpretation. The best thing to do is first check your state’s DOT laws to see how the GVWR and capacity ratings are interpreted, and also check the CDL requirements. Next, determine the weight of everything you plan to haul on a trailer—equipment plus attachments, accessories and options. Then, look for a trailer with a GVWR higher than the weight of everything you will be hauling. If your state recognizes weight transfer to the tow vehicle, you can take that into consideration, but if your state does not, then make sure the trailer you purchase or use has enough GVWR or, in some cases, GAWR to haul your equipment.
Use these formulas to determine your GAWR and GVWR:
GAWR = sum of all tire capacities, or the sum of all axle capacities
This formula is considered the GVWR on some manufacturer’s trailers. But other manufactures might use this formula for GVWR:
GVWR = Trailer Capacity + Trailer Weight
Payload capacity is then determined by one of these two formulas:
GVWR – Trailer Weight = Payload Capacity