Trailer Tire Sizes Explained: ST Codes, Load Range and Proper Fit
Trailer tire size is not a passenger-car decision. Special Trailer ST codes, load range, and the trailer's own loading information determine what may be fitted. The sidewall of a mounted tire can describe what is on the wheel today, but it does not prove that size is approved. Start with the trailer manufacturer's specification, then confirm load capacity, cold inflation pressure, wheel diameter, and how the complete designation fits the axle and wheel ratings.
Start With the Trailer’s Specified Tire Size
The correct trailer tire size begins on the trailer, not on a catalog page or a used wheel already sitting in the shop. Look for the tire and loading placard, the certification label, and the owner's manual, and keep those documents together with any manufacturer specification sheet. Write down the complete required size, the load specification, and the recommended cold inflation pressure as a single set. That record is the reference you will compare against every candidate tire and every inflation check before the trailer is loaded or towed.
An installed tire's sidewall can identify construction, section width, and inflation markings, but those markings describe the tire, not the trailer's approved fitment. A previous owner may have mounted a convenient size that still rolls, yet that does not make it the specified size. If the placard is missing, unreadable, or disagrees with the manual, stop guessing and contact the trailer manufacturer or a qualified trailer tire professional. Conflicting numbers need confirmation before you buy, mount, or inflate anything for the road.
Read Each Part of an ST Tire Size
A typical trailer tire size such as ST205/75R15 is read in a fixed order. ST is the ST tire designation for Special Trailer service, which is distinct from passenger-car service. The 205 is nominal section width in millimeters. The 75 is the aspect ratio, meaning sidewall height is seventy-five percent of that nominal width. R identifies radial construction, and 15 is the wheel diameter in inches. Those pieces together describe the tire's intended service family, width, profile, construction, and the rim diameter it is meant to match.
Wheel diameter in that string is the rim size the bead seats on, not the tire's overall outside diameter. Two tires that share a 15-inch wheel diameter can still differ in height, width, and clearance because section width and aspect ratio change the envelope. Trailer tire sidewall markings should be read as a complete ST service size, not as a passenger P-metric substitute. Some trailer tires use other size formats; those still require the manufacturer's specification rather than a conversion guessed from a similar passenger tire.
Understand Load Range and Rated Capacity
Load range describes a tire's load and inflation capability, but the letter itself is not a capacity in pounds. The rated load and the associated cold inflation pressure appear on the trailer tire sidewall, often with markings that distinguish single and dual applications. Use the figure that matches how the tire is actually mounted on the axle. Selecting by load range letter alone can miss a lower pound rating at the pressure you intend to run, or a dual-position rating that does not apply to a single tire.
A ply rating is a strength classification, not a count of the physical plies inside the carcass. Two tires with the same ply rating can still differ in construction and in the load they are allowed to carry at a given cold inflation pressure. Tire load capacity also does not raise the trailer's gross vehicle weight rating, gross axle weight rating, or the wheel's own load and pressure ratings. Those limits remain in force even if a stronger tire is installed, so the weakest approved rating in the system still governs what the trailer may carry.
Recognize Common Trailer Tire Size Markings
Readers may see ST175/80R13, ST205/75R14, ST205/75R15, and ST225/75R15 on existing equipment or in replacement stock. Those strings are examples of markings, not a ranking of popularity and not a map from trailer type to the right tire. The same size designation can be offered in more than one load range, so the size string alone is not enough to select a replacement. Record load range, load capacity, and the matching cold inflation pressure with the size, because two tires that look identical in width and wheel diameter can still be rated for different work.
Capture the full sidewall designation with a photograph or a written note taken from a safe standing position, without crawling beneath the trailer. Include the ST tire designation, size, load range, and any single or dual load markings so a professional can compare them to the trailer tire placard. Do not assign a size solely because the trailer is a particular type, length, or axle count. Those attributes do not replace the manufacturer's specified tire size, and similar trailers can be certified for different wheel and tire combinations.
Confirm Wheel Fit and Loaded Trailer Requirements
Approved fit also includes the wheel. Confirm approved wheel diameter and width, the wheel's load and pressure ratings, and valve suitability from manufacturer documentation or a qualified installer. A tire that can be forced onto a rim is not automatically a match for that wheel's ratings. Fender and suspension clearance must be checked for the specified tire dimensions through normal suspension movement, because a taller or wider envelope can rub under load even when the trailer sits level in the driveway.
Measured loaded axle weights help confirm that the chosen tires have enough load capacity for the work, but an axle total does not reveal uneven loading at individual wheels. Cargo, tongue weight, and side-to-side placement can load one tire well above its share of the axle figure. Do not change the specified tire size or load range without professional or manufacturer confirmation. The ability of a tire to mount on a wheel does not establish suitability for the trailer's ratings, clearance, or intended service.
Complete a Safe Check Before Towing
Before towing, confirm that the mounted tires match the approved size, that each has sufficient rated capacity for its position, that the wheels are suitable, and that inflation follows the trailer's specified cold inflation pressure. Sidewall pressure markings state the tire's own inflation capability; they are not a substitute for the trailer's inflation guidance. If the placard, the manual, and the sidewall disagree, obtain confirmation before the trip. Inflating to a sidewall maximum that the trailer never specified can still leave the combination outside its intended setup.
Walk around and look for bulges, cracking, exposed cords, irregular wear, and evidence of rubbing on fenders, frames, or suspension parts. Damage is a reason for professional assessment before travel, not a cue to keep driving and watch it. Include the spare in the same specification check so a roadside change does not introduce an unapproved size or load range. Missing markings, unexplained pressure loss, or uncertain fit all warrant inspection by a qualified trailer tire professional rather than a guess based on what still holds air.