Scientifically it makes sense, however, I don't believe the towing capacity is rated for an empty vehicle. Correct me if I'm wrong though.
OK, you asked for it.
- Passenger loads directly reduce payload capacity
- Payload capacity is basically the sum of all items (and persons) inside the vehicle that apply weight to the vehicles axles. EVERYTHING reduces it, including all the junk in your trunk, etc etc.
- Most average midside cars built today have total payload capacities in the 800 to 1100# range, so 4 fat people in the car and it's already legally overloaded sitting in your driveway. A surprising number of light duty "grocery getter" spec pickup trucks are only in the 1000-1500# cargo capacity range based on their GVWR minus their curb weight. (This is NOT to be mistaken with "tow capacity" ranges. This is where people get confused, they are NOT the same!)
- Trailer tongue weight is considered "Payload" since it is weight that is directly transferred to the vehicles axle(s).
So, a midsize car, say, a
Ford Fusion, Honda Accord, and Mazda6 with a payload capacity of 850#, well, that doesn't go very far if you have much going on. So, 100 pounds worth of stuff in your trunk (for example) leaves only 750# of payload capacity. Say you have 3 people total in the car at an average of 150# each for a total of 450# - now you only have 300 pounds of payload capacity left. Say you have 4 200# people in the car instead, along with that 100# of stuff in the trunk...and blammo, you're now technically overloaded and have zero capacity left for that "lightweight" trailer with the 150# of tongue weight.
Do the math again on a trailer with a 900# cruiser on it, the trailer itself weighs, say...750 pounds (rough weight for a steel single place sled trailer) for a total of 1650#. 15% of that weight (standard safe tongue weight for a tag trailer) = 247# of tongue weight.
Now that vehicle with a 850# payload capacity and 100# in the trunk only has 500# left for passengers.
So, doing the math again for a 1000# tandem deck trailer (again, using the snowmobile trailer example), and 2 800# cruisers, your trailer now weighs 2600# total, with a tongue weight of roughly 400 pounds. At that point you're not only exceeding the tongue weight capacity of most small to midsize cars anyways, but you've used half the payload capacity of the entire car before anything else, much less the driver and passengers.
Anyhow, like I said, it's complicated. Where it really gets interesting is when you start getting into pickup trucks - many people have found out the hard way that their "big burly" half ton truck that has a 10,000 pound tow rating can't actually haul a most fifth wheel or gooseneck trailers at all since all that "capacity" is only for tag trailers - the rear axle simply doesn't have the capacity to carry the much higher (25%) pin weights of a GN/5th. The aforementioned "grocery getter" spec pickup truck with a 1200# payload capacity, well, even worse yet - 4 big 200# passengers, 200# of camping gear, 2 cases of beer and some firewood...and you've maxed out the GVWR - hope you didn't plan on hooking up ANY trailer, much less a 5'th wheel camper.
