Understanding Trailer Tire Sizes and Ratings
Trailer tire sizes use the ST designation meaning Special Trailer, distinguishing them from passenger car (P) and light truck (LT) tires. A typical trailer tire code like ST225/75R15 indicates 225 millimeter width, 75 percent aspect ratio, radial construction, and 15 inch rim diameter. Trailer tires add load range letters C, D, or E indicating ply rating and maximum inflation pressure. Load range D means eight ply rated construction with 65 PSI maximum pressure and approximately 2,540 pound capacity. Understanding these codes ensures proper tire selection for safe towing within trailer and vehicle capacity limits.
Trailer tire sizing differs from automotive tires in critical ways. ST tires have stiffer sidewalls, heat resistant compounds, and construction designed for sustained heavy loads without steering input. Never substitute passenger car or light truck tires on trailers, as they lack the reinforcement and heat dissipation required for trailer service. Trailer tires operate at higher inflation pressures, typically 50 to 80 PSI compared to 30 to 35 PSI for passenger tires. This high pressure supports heavy loads and prevents excessive sidewall flexing that generates heat and leads to blowouts during highway towing.
ST Designation and Trailer Specific Construction
The ST prefix indicates Special Trailer service, meaning the tire is engineered specifically for trailer applications. ST tires have reinforced sidewalls that resist swaying and bouncing typical of trailer dynamics. The carcass construction handles vertical loads better than automotive tires designed for steering forces and cornering. ST tires use heat resistant rubber compounds to withstand sustained highway speeds under heavy loads without overheating. Tread patterns prioritize straight line stability and load carrying over steering responsiveness.
Using passenger car or light truck tires on trailers creates serious safety hazards. Automotive tires have flexible sidewalls for ride comfort and steering response, causing excessive sway and bouncing on trailers. They overheat quickly under trailer loads, leading to tread separation and blowouts. Passenger tire load ratings are calculated for vehicles with suspension systems and weight distribution trailers lack. Always use ST designated tires on trailers, never P or LT tires, regardless of apparent size matches or load ratings. The differences in construction and intended use make substitution dangerous.
Tire Width and Aspect Ratio
Trailer tire width ranges from 145 millimeters for tiny utility trailers to 235 millimeters or wider for heavy RVs and cargo trailers. Width affects load capacity and stability, with wider tires supporting more weight and providing better tracking. Common widths are 175, 205, 225, and 235 millimeters. Aspect ratio for trailer tires typically ranges from 70 to 80, higher than sport car tires which use 50 to 60. The taller sidewalls absorb road impacts and provide load cushioning for trailers without independent suspension.
A 75 aspect ratio means sidewall height equals 75 percent of tire width. For a 225 millimeter wide tire, sidewall height is 169 millimeters. This creates a tall profile that stabilizes trailers by raising the center of gravity slightly and providing more air volume for heat dissipation. Lower aspect ratios create stiffer ride and less air volume, unsuitable for most trailer applications. Changing aspect ratio by more than five percent alters overall diameter, affecting ground clearance and speedometer accuracy if the trailer has brakes with speed sensors.
Radial vs Bias Ply Construction
Trailer tires come in radial (R) and bias ply (D or dash) construction. Radial tires have steel belts running perpendicular to the direction of travel, creating flexible sidewalls and stable tread. Radials provide better heat dissipation, longer tread life, and improved fuel economy. They are now standard on most new trailers and are recommended for highway use. Radial construction handles heat buildup better during extended towing, reducing blowout risk.
Bias ply tires have crisscrossed nylon or polyester cords at angles, creating stiffer sidewalls. Bias tires are less expensive and more resistant to sidewall damage from curbs or off road use. They handle overload conditions better without catastrophic failure, making them popular for utility trailers used on construction sites or farms. However, bias tires generate more heat at highway speeds and wear faster than radials. For RV and travel trailers, radials are strongly recommended. For low speed utility use, bias tires remain acceptable and cost effective.
Rim Diameter Sizing
Trailer tire rim diameters range from 12 inches for small utility trailers to 16 or 17.5 inches for large RVs. Common sizes are 13, 14, 15, and 16 inches. Larger diameter wheels reduce tire rotation speed for a given road speed, generating less heat. They also provide more load capacity at equivalent tire width and aspect ratio. A 15 inch tire supports more weight than a 13 inch tire of the same width due to increased air volume and structural integrity.
Rim diameter must match exactly between tire and wheel. Never mount a 14 inch tire on a 15 inch wheel or vice versa, as the bead will not seat properly. Trailer wheels also have specific bolt patterns, typically five or six lug, and load ratings that must match or exceed trailer capacity. When replacing trailer tires, verify both tire and wheel load ratings meet or exceed your trailer's gross vehicle weight rating (GVWR) divided by the number of tires.
Load Range and Ply Rating Explained
Load range is a letter code indicating tire strength and load capacity. Range C is six ply rated with 50 PSI maximum pressure, suitable for light trailers. Range D is eight ply rated at 65 PSI, the most common range for travel trailers and cargo trailers. Range E is 10 ply rated at 80 PSI for heavy fifth wheels and commercial trailers. Each load range supports progressively higher weight, with specific capacities listed in tire load charts based on inflation pressure.
Ply rating no longer refers to actual fabric plies in modern tires but indicates equivalent strength. A six ply tire may have only two physical plies of stronger materials achieving six ply equivalent strength. Higher ply ratings mean stiffer sidewalls, higher inflation pressures, and greater load capacity. Always match or exceed the OEM load range when replacing trailer tires. Using lower load range tires than specified creates overload conditions, excessive heat, and blowout risk even if the trailer weight seems acceptable.
Load Capacity and Inflation Pressure
Trailer tire load capacity depends on both tire size and inflation pressure. Each tire has a maximum load at maximum pressure, printed on the sidewall. For example, an ST225/75R15 load range D tire may be rated for 2,540 pounds at 65 PSI. Running lower pressure reduces capacity proportionally. At 50 PSI, the same tire might support only 2,200 pounds. Always inflate trailer tires to maximum sidewall pressure, not the vehicle placard pressure used for cars.
To calculate required tire capacity, divide trailer gross weight by number of tires and add 10 to 15 percent safety margin. A 6,000 pound trailer with dual axles has four tires, so each tire must support 6,000 divided by four equals 1,500 pounds minimum. Adding 15 percent safety margin requires 1,725 pound capacity per tire. This accounts for weight distribution imbalances, loading variations, and dynamic forces during towing. Select tires with capacity exceeding this calculated minimum at maximum inflation pressure.
Speed Ratings and Towing Speeds
Trailer tires typically have lower speed ratings than automotive tires. Most ST tires are rated for M (81 mph) or L (75 mph) maximum speeds. Some specialty trailer tires reach N (87 mph). These ratings reflect heat buildup limitations during sustained high speed towing. Exceeding speed ratings causes excessive heat, accelerating wear and increasing blowout risk. Towing at or below 65 mph significantly extends trailer tire life and safety.
Trailer tire speed limitations stem from their construction and operating conditions. Trailers lack powered axles and suspension dampening, causing tires to bounce and flex more than automotive tires. This generates heat rapidly at high speeds. Trailer tires also operate at higher loads relative to their size than car tires, contributing to heat buildup. For safety and tire longevity, limit towing speed to 65 mph regardless of speed rating, and reduce speed to 55 mph on hot days or when fully loaded.
Alternative Sizing Formats
Some older trailers use numerical or alpha numeric sizing like 5.30-12 or 20.5-8. These formats indicate tire width and rim diameter in inches, with the first number being approximate tire width and second number rim diameter. For example, 5.30-12 means approximately 5.3 inches wide on a 12 inch rim. Converting these sizes to modern metric requires cross reference charts, as direct equivalents don't always exist.
When replacing tires on older trailers, consult tire dealers or manufacturers for proper metric equivalents. In some cases, changing to metric sizing requires different wheels if modern tire beads don't match vintage rim profiles. Document the existing tire dimensions and load capacity, then find metric replacements meeting or exceeding these specifications. Upgrading from bias to radial construction often requires verifying wheel and brake clearances, as radial tires may have different outside diameters than bias tires of equivalent sizing.
Practical Tips for Trailer Tire Selection
Before purchasing trailer tires, determine your trailer's GVWR from the VIN plate or documentation. Calculate required per tire capacity by dividing GVWR by tire count and adding 15 percent. Verify current tire size on sidewalls and in owner's manual. Select replacement tires matching size, load range, and speed rating or better. Inflate to maximum sidewall pressure and check monthly, as trailer tires lose pressure faster when sitting unused.
Finally, inspect trailer tires before each trip for cracking, bulges, and tread wear. Replace tires older than five to seven years regardless of tread depth, as rubber compounds deteriorate with age. UV exposure accelerates aging, so cover trailers when stored outdoors or use tire covers. A trailer tire size guide with load capacity charts ensures you select proper tires for safe towing, preventing blowouts and maintaining control while hauling cargo, boats, or recreational vehicles.