The question of how much air pressure to put in trailer tires is one of the most frequently misunderstood aspects of safe towing. Unlike passenger vehicle tires, which often have a range of recommended pressures based on the load, trailer tires operate under a different set of rules. Maintaining the correct inflation pressure, measured in pounds per square inch (PSI), directly influences the tire’s ability to carry its rated weight and withstand dynamic forces. Understanding these specific requirements is necessary for ensuring both towing efficiency and the longevity of the equipment.
Locating the Maximum Cold Inflation Pressure
The most direct answer to determining the correct PSI is to locate the “Maximum Cold Inflation Pressure” stamped directly onto the tire’s sidewall. This figure represents the pressure needed for the tire to achieve its full load-carrying capacity as determined by the manufacturer. For Special Trailer (ST) tires, the maximum pressure listed on the tire itself is the generally accepted target.
Measuring “cold inflation pressure” means checking the air pressure when the tires have not been driven for at least three hours or have traveled less than a mile. Driving causes friction and heat, which naturally increases internal pressure, meaning a reading taken after driving will be falsely high. Checking the pressure must be done before every tow, using a reliable gauge. Failing to meet this maximum pressure means the tire cannot support its full rated load, which compromises towing safety.
Why ST Tires Require Maximum PSI
Trailer tires are designated with the prefix “ST,” which stands for Special Trailer, indicating they are constructed differently than standard Passenger (“P”) or Light Truck (“LT”) tires. The design of an ST tire prioritizes high load capacity and minimal sidewall flex, achieved through specific internal construction and rubber compounds. This specialized structure is engineered to counteract the unique side-to-side forces, or scrubbing, that trailers experience during turns.
To fully engage the tire’s load-bearing capability and stabilize the towing dynamics, the tire must be inflated to the full pressure indicated on the sidewall. This pressure corresponds to the tire’s Load Range designation, which directly correlates to the maximum weight the tire can safely support. Unlike P-Metric tires, ST tires must be inflated to their maximum specified PSI to ensure the full internal tension is maintained across the casing plies. Operating an ST tire below its maximum pressure prevents the tire from achieving its maximum stability and load capacity.
Safety Risks of Incorrect Inflation
The most prevalent and dangerous issue for trailer tires is under-inflation, which creates excessive heat buildup within the tire structure. Low pressure allows the sidewalls to flex dramatically, causing internal friction that rapidly raises the operating temperature. This uncontrolled heat accumulation leads to the breakdown of rubber compounds and the bond between the internal steel belts and plies. The resulting structural failure is the primary cause of sudden, catastrophic blowouts on the highway.
Operating a tire at a pressure even 20% below its maximum rating significantly increases the risk of premature failure and tread separation. Over-inflation also presents a risk, primarily one of accelerated wear and reduced traction. Excessively high pressure causes the center of the tread to bulge slightly, concentrating the load onto a smaller contact patch. This reduced footprint negatively affects stability and braking performance.