Common trailer axle capacities are: 3500 lbs - 5 bolt 15" wheels, 5200 lbs - 6 bolt 15" wheels, 6000 lbs - 6 bolt 16" wheels or 7200 lbs - 8 bolt 16" wheels. Even with the recommended 10-15% tongue weight, the car being carried and the trailer itself would need to be very light to be safely carried by a single axle. This leaves no load margin in hot weather/high speeds where tires usually fail due to heat buildup in the carcass of the tire. (Seen any single axle trailers stranded on the highway in the summer?) ? Also, a pair of automotive tires and wheels typically used on homebuilt trailers don't even come close to the capacities listed above. In the 50s and 60s homebuilt trailers were very common. For engineering and liability concerns, not so much now days. As modelers, we can get away with anything ? However, I have seen many examples in the real world where "if it fits I can haul it" has lead to spectacular failures. Tongue weight is critical to handling. That is why boat trailer axles are located further aft ( engine weight is at the rear). I once witnessed a really nice 69 Nova leave the drag strip loaded about 2 feet further aft than it should have been on a tandem avle trailer behind a 1 ton dually. When the rig hit 55 the trailer started to sway so bad that it went sideways down both shoulders before the driver was able to get it stopped. Thankfully no damage except to the driver's shorts!