Big deal. A tire is a tire, right ? Wrong. As a matter of fact there's a big difference between trailer tires and your own car tires. First of all the side walls in any passenger vehicle tire are made to flex. A vehicle's tires are designed for ride comfort. Your trailer is made to haul loads and follow behind. Sidewall flexing on trailer tires is not good at all, especially those with a high center of gravity like an enclosed travel trailer. it's the primary cause of trailer sway.
So then, before you decide to put a cool set of 15 inch radial tires on your trailer, just remember, flexible sidewalls can accentuate trailer sway. Nobody wants trailer sway. The stiffer sidewalls and higher operating pressures common with Special Trailer (ST) designated tires help reduce trailer sway. Remember; Your tow vehicle is a leader, which means traction is a key focus in the design of its tires. Traction allows your tow vehicle to accelerate down the road, turn around the corner and brake to a stop. Your trailer is a follower.
Trailers will be more stable and pull better on tires designed specifically for trailer use. Since Special Trailer (ST) tires are constructed with heavier duty materials, they are tougher than typical passenger vehicle tires. This is a plus because trailer suspension systems are generally stiffer and less sophisticated than automotive suspension systems.
Industry standards dictate tires with the ST designation are speed rated to 65 MPH (104 km/h) under normal inflation and load conditions. One last thing:
Do not exceed the wheel's maximum rated pressure. If the maximum pressure for the wheel prohibits the increase of air pressure, then maximum speed must be restricted to 65 mph (104 km/h).
The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire. Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.
( Credits: tirerack.com )