Trailer Tire Size Chart

01The chart

These figures come from Tredit Tire & Wheel's published specifications for the Trailer King II ST line, which covers the full range from Load Range C through F in one consistent table. Every size in it carries an M speed rating — 81 mph — which is worth noting given how often trailer tires are still described as universally 65 mph tires.

Capacities differ modestly between manufacturers in the same size and load range. The clearest example: ST235/80R16 in Load Range E is published at 3,520 pounds by Trailer King and 3,420 pounds by Goodyear for the Endurance. That is a 100-pound difference per tire, or 400 pounds across a tandem axle, and it is enough to matter if you are close to your rating. Read the sidewall of the tire you are actually buying.

02How to read an ST tire size

Take ST205/75R15 apart. ST means Special Trailer — the tire is built for a trailer axle and is not rated for use on a powered or steered vehicle. 205 is the section width in millimeters. 75 is the aspect ratio: the sidewall height is 75 percent of the section width. R means radial construction. 15 is the wheel diameter in inches.

The letter before the wheel diameter is the one people miss. ST205/75R15 is a radial. ST205/75D15 is a bias-ply tire in the same nominal size — the D stands for diagonal construction, and it is a different tire with different heat behavior and a lower speed capability. If you are replacing a set and the price looks unusually good, check whether you are comparing a radial to a bias-ply.

After the size comes the service description — something like 107M or 117N. The number is the load index and the letter is the speed rating. Load index 107 is 2,149 pounds, 117 is 2,833 pounds. M is 81 mph and N is 87 mph. Between the load range letter on the sidewall and the service description you have everything you need to know about what the tire can do.

03Finding the right size for your trailer

The authoritative source is the trailer's certification label, not the tire currently on it. Manufacturers stamp a metal or foil plate — usually on the left front of the frame, near the coupler or on the A-frame — listing the trailer's GVWR, its axle ratings, and the tire size, load range and cold inflation pressure it was built around. A previous owner may have fitted whatever was cheap; the plate has not changed.

If the label is missing or unreadable, work from the axle rating. Find the axle capacity stamped on the axle tube or in the trailer's paperwork, divide by the number of tires on that axle, and choose a tire whose maximum load meets or exceeds the result. On a tandem-axle trailer, do the math per axle rather than dividing the trailer's GVWR by four, because loads rarely sit evenly front to rear.

Give yourself margin. Sizing a tire to exactly the axle rating means every tire is at 100 percent of its capacity the moment the trailer is loaded to its rating, with nothing left for an uneven load, a hot day, or the pressure being a few psi down. A reserve of 10 to 20 percent is a cheap insurance policy at these prices.

04Can you change your trailer's tire size?

Sometimes, and the constraints are physical before they are anything else. Overall diameter changes fender clearance and, on a tandem axle, the clearance between the two tires. Going from ST205/75R15 at 27.1 inches to ST225/75R15 at 28.3 inches adds over an inch of diameter and roughly three-quarters of an inch of section width, which is enough to touch a fender on a trailer that was built tight.

Section width also has to fit the wheel. Trailer wheels are narrow, and a wider tire on a wheel that is too narrow pinches the tread profile and wears the shoulders. If you go up in width, check the tire's approved rim width range before assuming the existing wheels will work.

The one direction that is never acceptable is down in capacity. Whatever you fit has to meet or beat the load rating on the certification label at the pressure you are going to run. The easiest way to gain capacity without touching diameter at all is to move up a load range in the same size — ST205/75R15 goes from 1,820 pounds in Load Range C to 2,150 in Load Range D, with the pressure going from 50 to 65 psi.

05Pressure goes with the load range, not the truck

The inflation column in the chart is not a suggestion and it is not the same logic as a car tire. ST tires are designed to operate at or very near their maximum cold pressure, because a trailer axle sits close to its rated load whenever the trailer is loaded and has none of the suspension compliance a vehicle has.

Practically: a Load Range C trailer tire runs 50 psi cold, a Load Range D runs 65, a Load Range E runs 80, and a Load Range F runs 95 to 100 depending on the size. Set them cold, before the first mile of the day, and set the spare too. Never use the tow vehicle's door-jamb pressure on the trailer — that number belongs to the truck.

If you also need to sort out what the truck's tires should be running while towing, that is a separate calculation: see tire pressure for towing.

06Getting the size you need in West Georgia

The small sizes — 13-inch and 14-inch utility and boat trailer tires — are the ones local shops are most likely to have on the shelf. Sixteen-inch Load Range E and F sizes for equipment and gooseneck trailers are more often ordered in, usually within a day or two.

This directory does not track trailer tires as a separate service, and no shop in it lists them as its own line item — but 73 of the 75 shops listed across Carroll, Douglas, Paulding, Haralson and Heard counties sell and mount tires. Call with the size and load range off your certification label rather than describing the trailer, and ask for the date code on whatever they would install.

RimsAndTires is a free directory, not a tire seller. Send us the size off your trailer's plate and we will point you at a local shop that has it.

Trailer tire size chart — common ST radial sizes, load ranges and capacities