Trailer Tire Sizes: ST Load Chart, Load Ranges and Diameters
For anyone replacing or upgrading tires on a utility, boat, car-hauler or camper trailer. Trailer tires are sized like car tires but rated very differently, and the load range letter matters as much as the size.
ST225/75R15 E means a Special Trailer tire 225 mm (8.9 in) wide with a 75% aspect ratio, radial, for a 15-inch wheel, in load range E (10-ply rated). It stands about 28.3 in (719 mm) tall and carries up to 2,830 lb (1,284 kg) at 80 psi. ST tires without a speed symbol are rated for 65 mph (105 km/h).
- Diameter
- 28.3 in (719 mm)
- Max load (E)
- 2,830 lb at 80 psi
- Load range D
- 8-ply, 65 psi
- Default speed
- 65 mph (105 km/h)
On this page
How to read a trailer tire size
Trailer tires use the same layout as car tires, with two differences: the ST prefix and the load range letter at the end.
| Part | Meaning |
|---|---|
| ST | Special Trailer service. Built with stiffer sidewalls to resist sway and carry heavy static loads; not for use on cars or trucks. |
| 225 | Section width in millimeters (8.86 in) |
| 75 | Sidewall height is 75% of the width: 169 mm (6.6 in) |
| R | Radial construction. A dash (ST225/75D15 or ST225/75-15) means bias-ply. |
| 15 | Wheel diameter in inches |
| E | Load range E: 10-ply rated, 80 psi maximum |
Some newer ST tires also carry a load index and speed symbol (for example 117/112M, where 117 is the single-tire load index, 112 the dual-tire index, and M = 81 mph). Older and many current ST tires have no speed symbol at all.
Trailer tire load range chart
Load range has replaced ply count as the strength rating. A modern load range E tire does not literally have ten plies; it is as strong as a ten-ply bias tire was.
| Load range | Ply rating | Max pressure | Typical trailers |
|---|---|---|---|
| B | 4 | 35 psi (241 kPa) | Small utility and jet-ski trailers |
| C | 6 | 50 psi (345 kPa) | Utility, small boat and pop-up camper trailers |
| D | 8 | 65 psi (448 kPa) | Boat trailers, travel trailers, landscape trailers |
| E | 10 | 80 psi (552 kPa) | Travel trailers, car haulers, equipment trailers |
| F | 12 | 95 psi (655 kPa) | Heavy fifth-wheels, gooseneck trailers |
| G | 14 | 110 psi (758 kPa) | Heavy fifth-wheels, equipment trailers |
ST tire size and load chart
Maximum single-tire loads at the maximum pressure for each load range, from the standard TRA load tables. Diameters are calculated from the size. Running below maximum pressure lowers the load capacity.
| Size | Load range | Max load | Pressure | Diameter |
|---|---|---|---|---|
| ST175/80R13 | C | 1,360 lb (617 kg) | 50 psi | 24.0 in (610 mm) |
| ST185/80R13 | C / D | 1,480 / 1,710 lb | 50 / 65 psi | 24.7 in (626 mm) |
| ST205/75R14 | C / D | 1,760 / 2,040 lb | 50 / 65 psi | 26.1 in (663 mm) |
| ST215/75R14 | C | 1,870 lb (848 kg) | 50 psi | 26.7 in (678 mm) |
| ST205/75R15 | C / D | 1,820 / 2,150 lb | 50 / 65 psi | 27.1 in (689 mm) |
| ST225/75R15 | C / D / E | 2,150 / 2,540 / 2,830 lb | 50 / 65 / 80 psi | 28.3 in (719 mm) |
| ST235/80R16 | C / D / E | 2,600 / 3,000 / 3,420 lb | 50 / 65 / 80 psi | 30.8 in (782 mm) |
| ST235/80R16 | F / G | 3,740 / 4,080 lb | 95 / 110 psi | 30.8 in (782 mm) |
| ST235/85R16 | E / F / G | 3,640 / 3,960 / 4,400 lb | 80 / 95 / 110 psi | 31.7 in (806 mm) |
| ST225/90R16 | C / D / E | 2,680 / 3,200 / 3,520 lb | 50 / 65 / 80 psi | 31.9 in (811 mm) |
Tandem trailers with tires side by side (dual fitments, rare on trailers) use the lower dual rating. Individual tire models can be rated slightly differently, so confirm with the sidewall, which always shows the maximum load and pressure.
Height difference between common 15-inch sizes
Changing size alters the trailer's ride height by half the diameter difference, and the tire needs clearance to the fender and, on tandems, to the tire next to it.
| From | To | Diameter change | Ride height change | Width change |
|---|---|---|---|---|
| ST205/75R15 | ST225/75R15 | +30 mm (+1.2 in) | +15 mm (+0.6 in) | +20 mm (+0.8 in) |
| ST225/75R15 | ST235/75R15 | +15 mm (+0.6 in) | +8 mm (+0.3 in) | +10 mm (+0.4 in) |
| ST205/75R15 | ST235/75R15 | +45 mm (+1.8 in) | +23 mm (+0.9 in) | +30 mm (+1.2 in) |
On a tandem axle, check the gap between the front and rear tire: a 1.2 in taller tire closes it by 1.2 in. Most builders leave at least 2 in.
How to choose trailer tire size and load range
- Find the trailer's GVWR and axle ratings. They are on the VIN plate, usually on the tongue or the front of the left side.
- Work out the load on the tires. Loaded weight minus tongue weight. Tongue weight is usually 10-15% of the trailer weight for bumper-pull trailers and 15-25% for fifth-wheels and goosenecks.
- Divide by the number of tires. A 7,000 lb travel trailer with 12% (840 lb) on the hitch puts 6,160 lb on four tires: 1,540 lb each.
- Add a margin. Uneven loading, turning scrub on tandems and heat all add stress. A 10-20% reserve is widely recommended: 1,540 × 1.2 = 1,848 lb, so ST205/75R15 load range C (1,820 lb) is marginal, and load range D (2,150 lb) or ST225/75R15 is the safer choice.
- Check the wheels. Each wheel's stamped load and pressure ratings must be at least equal to the tire's.
ST load ratings only apply at the stated pressure. An 80 psi load range E tire run at 50 psi carries about the same as a load range C tire. Check pressure cold, before each trip; trailer tires lose pressure while stored.
Trailer tire speed ratings: the 65 mph rule
Under the TRA standard, ST tires without a speed symbol are rated at 65 mph (105 km/h) at full load. Goodyear's TRA-based guidance for speeds of 66-75 mph is to add 10 psi to the recommended cold pressure, without counting on any extra load capacity, and never to exceed the wheel's or valve's maximum pressure. If that is not possible, 65 mph stays the limit.
Many current radial ST tires carry a speed symbol, most often L (75 mph, 120 km/h) or M (81 mph, 130 km/h). Even then, check the trailer maker's maximum speed and local towing speed limits, which are often lower (several US states set 55 mph limits for vehicles towing trailers).
Small trailer and bias-ply sizes
Small utility, boat and jet-ski trailers often use bias tires sized in inches. They follow different naming patterns:
- 4.80-12 or 4.80x12: about 4.8 in section width on a 12-inch wheel. Often sold as 4.80-12 / 4.00-12 because the same tire fits both rim sizes.
- 5.30-12 and 5.70-8: the same convention for slightly wider or smaller tires.
- 20.5x8.0-10: flotation style: about 20.5 in tall, 8 in wide, on a 10-inch wheel. Common on personal watercraft and small boat trailers.
- ST175/80D13 or ST205/75D15: bias-ply versions of metric ST sizes, with the D in place of R.
Small trailer wheels use their own bolt patterns: 4 on 4 in (4x101.6 mm) on 8-12 inch wheels, 5 on 4.5 in on most 13-15 inch wheels, and 6 on 5.5 in or 8 on 6.5 in on 16-inch wheels for heavier trailers. See the wheel size guide for how to measure a pattern.
Spare tires and storage
- Carry a matching spare on its own wheel, at the same pressure as the road tires. Tandem trailers can limp on three tires only at reduced load and speed.
- Storage: park on wood or plastic rather than bare ground, cover the tires from UV, and if the trailer sits for months, raise it on stands or move it every few weeks to avoid flat spots.
- Mixing: keep the same size, construction (radial or bias) and load range on each axle.
For how car and truck tire sizes compare with ST sizes, see the tire size chart. If you are hauling with a pickup, the truck bed size guide covers bed dimensions and payload.
Frequently asked questions
What does the letter D on a trailer tire mean?
D is the load range. It means an 8-ply-rated casing with a maximum cold pressure of 65 psi. An ST205/75R15 in load range D carries 2,150 lb at 65 psi, compared with 1,820 lb at 50 psi in load range C.
Do you need special rims for load range D or E trailer tires?
The wheel must be rated for at least the tire's maximum load and pressure. Both are stamped on the wheel (for example a maximum load in pounds and a maximum cold psi). Many light steel trailer wheels are rated for 50 or 65 psi, which is too low for an 80 psi load range E tire. Snap-in rubber valves are typically rated to 65 or 80 psi, so high-pressure tires may need metal clamp-in valves.
What is the outside diameter of a 15-inch trailer tire?
It depends on the size: ST205/75R15 is about 27.1 in (689 mm) and ST225/75R15 about 28.3 in (719 mm).
Can I put car or light-truck tires on a trailer?
LT tires are acceptable if their load rating at the pressure you run covers the trailer's axle load. Passenger (P-metric) tires used on a trailer must be derated: divide the sidewall load by 1.10. ST tires should never be fitted to a car or truck.
How often should trailer tires be replaced?
Trailer tires usually age out before they wear out. A common guideline from trailer-tire makers is three to five years of service, and few should be kept beyond about six years from the DOT date code even with full tread. Cracking in the sidewall or between tread blocks is a sign to replace them sooner.
How much does a trailer tire weigh?
Roughly 20-25 lb for a 13-14 inch radial, 25-35 lb for ST205/75R15 and ST225/75R15 depending on load range, and 40-50 lb for 16-inch load range E to G tires. A steel wheel adds about 15-25 lb. Exact weights are on each maker's spec sheet.