Marmuletka/Shutterstock Trailer tires are of a different breed than passenger car tires. Also known as Special Trailer (ST) tires, they are built stiffer to enable higher load capacity, hold more air pressure, and mitigate sidewall flexing, which helps reduce trailer sway. They are often constructed from tougher steel belts and substantial wires for maximum strength. Besides that, trailer tires are constructed from compounds with more antioxidants to resist the damaging effects of UV rays, sunlight, and ozone that may lead to premature dry rot. Unlike car or SUV tires with softer, more flexible sidewalls designed to absorb bumps, trailer tires are designed to do the opposite. That doesn't make them inferior, though, since a stiffer sidewall mitigates compression or deflection, enabling them to literally stand tall under load. Just as an underinflated tire accumulates more heat, the rigid construction of trailer tires helps keep them cooler on long hauls. ST tires also have shallower treads than car tires — where new passenger car tires have average tread depths of 10/32 to 11/32 inches, trailer tires start at around 8/32 of an inch, and that's not a bad thing. Shallow treads equate to less rolling resistance and tread squirm while under load. The contact patch or tread footprint – the part of the tire that's touching the road at any given moment — would typically twist and squirm as it steers and rolls down the road in response to the vehicle's weight. Trailer tires are designed for carrying loads and not to steer or power the vehicle, and their robust sidewalls that resist deflection make them less likely to buckle, compress, or squirm. Trailer tires come in a different size group Canart7/Getty Images Trailer tires may have stiffer sidewalls and tougher construction overall, but they typically come in smaller sizes than passenger (P) and light truck (LT) tires, with taller aspect ratios and narrower widths to manage heavy-duty hauling and towing. The aspect ratio is just one of many numbers and codes written on a tire's sidewall, and it refers to the height of the tire's sidewalls in relation to the tire's width. A lower aspect ratio means the tire's sidewall is shorter, as is the case with low-profile tires for sporty cars and crossovers. Skinnier tires with higher aspect ratios are more beneficial to trailers, since a narrower contact patch exerts more pressure per square inch and enables the tires to track better under heavy loads. Using tires that are wider than what the manufacturer recommends for your trailer could cause wandering or make unnecessary movements or vibrations that exert more stress on the trailer tongue. How to choose the right trailer tire Bruce W Smith/Getty Images Choosing the right trailer tire is no different from selecting the best tires for your four-wheeled vehicles. Besides going for the proper size, the load range is equally important. The load range is expressed by a letter and a number embossed on the sidewall and would typically denote the tire's maximum inflation pressure and ply rating. For instance, an ST tire with a C1 or C2 load range has similar 6-ply ratings, but while C1 has a maximum air pressure of 50 psi, C2 is rated at 35 psi. Another important factor to consider is the load index of the tire, which indicates the maximum load the tire can support. Similar to the load range, you can also determine the load index of a tire via the sidewall markings. A higher load index means a higher carrying capacity, and you can match the numbers to a specific weight by referring to a load index chart. As an example, a trailer tire with a load index of 60 has a carrying capacity of 551 pounds, while a load index of 100 can support up to 1,764 lbs. Pay attention to the speed rating when choosing trailer tires, too. Most ST tires have a maximum rating of 65 mph, but some newer tires can roll faster without compromising stability. ST tires with an L rating are good for 75 mph, while an M speed rating can go 81 mph.