Trailer Tire Sizes, Load Ranges and Bolt Patterns Explained
Buying trailer tires is straightforward once you know what the sidewall, the load range, and the wheel bolt pattern actually mean. This guide explains how to read sizes such as ST205/75R15, how load range differs from the number of plies you can count, how ST tires differ from LT tires, and how to measure 4-lug, 5-lug, and 6-lug patterns before you buy tires or a tire-and-wheel assembly.
ST trailer tires vs. LT and passenger tires
Tires marked ST (Special Trailer) are built for trailers. They typically have stiffer sidewalls than passenger or light-truck tires, which helps them carry load and resist sway when the trailer is being towed. They are not designed for the steering, camber, or drive torque of a car, truck, or SUV. Use ST tires on trailers when that is what the trailer maker specified, and do not install ST tires on a tow vehicle.
LT (Light Truck) tires are designed for trucks and SUVs that steer, brake, and drive. Their sidewalls are usually more flexible than ST tires. Some owners fit LT tires on trailers, but the load, inflation, and speed characteristics are not the same, and the trailer placard often calls for ST. If the trailer manufacturer specifies ST, stay with ST unless the manufacturer says otherwise. P-metric passenger tires are generally not a substitute for trailer service.
Do not mix ST and LT tires on the same axle, and do not mix tire sizes on the same axle. If you replace one tire, match the size, load range, and construction of the other tire on that axle, or replace them as a pair.
How to read a trailer tire size
Metric sizes such as ST205/75R15
Most highway trailer tires use a metric size. In ST205/75R15:
- ST means Special Trailer.
- 205 is the tire's section width in millimeters.
- 75 is the aspect ratio: the sidewall height is 75 percent of the width.
- R means radial construction. A D (for example ST205/75D15) means diagonal, or bias-ply, construction. A dash in older numeric sizes (4.80-12) also indicates a bias-ply tire.
- 15 is the wheel, or rim, diameter in inches. The new tire must match this number.
The same logic applies to other common trailer sizes, such as ST175/80D13, ST205/75D14, ST205/75R14, ST225/75D15, and ST225/75R15. The safest replacement is the same size shown on your current sidewall and on the trailer's tire placard or VIN label.
Numeric and flotation sizes
Smaller utility and boat trailers sometimes use a numeric size such as 4.80-12 or 5.30-12, or a flotation-style size such as 20.5x8.0-10. In those markings the leading numbers describe width and overall or section dimensions, and the last number is still the rim diameter in inches. Do not assume a metric size is a drop-in for a numeric size, even if the rim diameter matches. Use the size on the placard or the sidewall.
Load range and load capacity
Load range is a letter that indicates the tire's maximum load capacity and the inflation pressure that goes with that load. The "ply rating" you see next to it is a strength equivalent. On modern tires it is not a count of the actual number of body plies in the carcass, so a "6-ply rating" tire may not contain six physical plies.
| Load range | Common ply rating |
|---|---|
| B | 4-ply rating |
| C | 6-ply rating |
| D | 8-ply rating |
| E | 10-ply rating |
| F | 12-ply rating |
The number that actually matters is the maximum load, in pounds, molded on the sidewall, together with the pressure at which that load applies (for example, "Max load 1820 lbs at 65 psi"). To check a set of tires, add up the load capacity of every tire that will be on the ground and compare that total with the loaded weight of the trailer and its cargo. The total tire capacity must be at least that weight, and you must also stay within the trailer's GVWR (gross vehicle weight rating) on the trailer placard. Many owners choose a load range with some capacity to spare. Never exceed the maximum load or the maximum pressure molded on the tire, and never exceed the wheel's own load or pressure rating.
You can often move up in load range (for example from C to D) in the same size if the wheel is rated for the higher inflation pressure that the heavier-duty tire requires. Do not move down in load range below what the trailer manufacturer specified.
Radial or bias-ply?
Radial trailer tires (the "R" in ST205/75R15) generally run cooler, wear more evenly, and ride more smoothly on long highway trips. Bias-ply tires (the "D" in ST205/75D15, or a dashed numeric size) have a different, often stiffer, casing and are still common on utility trailers, especially at lower speeds or when a trailer sits for long periods.
Do not mix radial and bias-ply tires on the same axle. If you change construction, change both tires on that axle, and confirm that the size and load range still match the placard. A listing that shows ST205/75R15 is not the same tire as ST205/75D15; the rim diameter matches, but the construction does not.
How to measure a bolt pattern
If you are buying a tire already mounted on a wheel, the wheel has to fit your hub. A bolt pattern is written as the number of lug studs and the diameter of the circle that passes through their centers. For example, 5 on 4.5 (5x4.5, or 5x114.3 mm) means five studs equally spaced on a 4.5-inch circle. Common trailer patterns include 4 on 4, 5 on 4.5, 5 on 4.75, 5 on 5, 6 on 5.5, and 8 on 6.5.
4-lug, 6-lug, and 8-lug patterns
These patterns have a stud directly across from another stud. Measure from the center of one stud (or lug hole) to the center of the stud directly opposite. That measurement is the bolt-circle diameter.
5-lug patterns
A 5-lug pattern has no stud directly across from another, so a simple center-to-center measurement across the hub is not the bolt-circle diameter. A widely used field method is to measure from the center of one stud to the outer edge of the stud two positions away (the farthest stud). That approximation is usually close to the true bolt-circle diameter. A bolt-pattern gauge or a printed template is more precise if you are between two common sizes, such as 5 on 4.5 and 5 on 4.75. When you can, confirm the pattern from the trailer maker's documentation or from a marking on the existing wheel.
Other wheel fitment checks
Bolt pattern is not the only measurement. The wheel's center hole must clear the hub (or fit a matching center cap / hub-centric ring if one is used). The wheel's load rating should be at least as high as the tire's load, and the wheel must be rated for the inflation pressure you will use. Also check pilot diameter, offset or dish, and whether your trailer uses standard or a less common hub style. A wheel that bolts on but sits too far in or out can rub or overload the bearings.
Pressure, speed, and age
- Pressure: Check the pressure when the tires are cold, before you tow, using a gauge you trust. Use the inflation on the trailer placard when it is given, and do not go above the maximum pressure molded on the sidewall. Underinflation is a leading cause of trailer tire failure. Recheck after the first miles of a trip and whenever the trailer has sat unused.
- Speed: Many ST tires have a maximum speed stated on the sidewall or in the tire maker's data, often 65 mph, even when a speed letter also appears. Do not tow faster than the tire (and the trailer) is rated for.
- Age: Trailer tires often age out before the tread wears out. The last four digits of the DOT code are the week and year of manufacture (for example, 2324 means the 23rd week of 2024). Inspect older tires for cracking, dry rot, bulges, and damaged beads, and follow the tire maker's age guidance. Replace tires in pairs on the same axle when the remaining tire is much older or worn unevenly.
How to choose a replacement
- Read the size, load range, and maximum load from the current sidewall, and compare them with the trailer placard.
- Note whether the tire is radial (R) or bias-ply (D or a dashed numeric size).
- If you are buying a complete assembly, measure the bolt pattern, and confirm the rim diameter, center hole, and wheel load rating.
- Match size and construction exactly. Keep load range the same or, if the wheel allows the higher pressure, consider a higher load range. Do not drop below the specified load range.
- Add up the load capacity of the tires you will run and confirm it covers the loaded trailer weight without exceeding GVWR or the wheel ratings.
- If any measurement is unclear, contact us with a photo of the sidewall, the placard, and — for an assembly — the bolt pattern and any markings on the existing wheel.
Common mistakes
- Buying ST205/75R15 when the trailer is fitted with ST205/75D15, or the other way around.
- Treating "6-ply" as a literal ply count, or ignoring the max-load figure on the sidewall.
- Measuring a 5-lug pattern as if a stud sat directly opposite, and ordering the wrong bolt circle.
- Mixing ST and LT, or radial and bias-ply, on the same axle.
- Fitting a higher-load-range tire on a wheel that is not rated for the higher inflation pressure.
- Installing ST tires on the tow vehicle, or passenger tires on a trailer that calls for ST.
Shop trailer tires on shopmanu.com
Browse trailer tire & wheel assemblies and the full tires & wheels collection. Listings show size, construction, and other fitment details; always confirm those against your sidewall and placard. Examples currently listed include a set of two ST205/75R15 radial assemblies, a set of two ST205/75D15 bias-ply assemblies, a set of two ST225/75D15 assemblies, a set of four ST175/80D13 assemblies, a set of four ST205/75D14 assemblies, and a set of four ST225/75R15 radial assemblies.
Ordering, shipping and returns
Manu is the seller of record for every order placed on shopmanu.com, and we are your first point of contact for questions about an order. Shipping charges are calculated and shown at checkout before you pay, and delivery dates are estimates rather than guarantees. Full details are in our Shipping Policy. For how orders are sold and fulfilled, see how Manu works.
Eligible items can be returned within 30 calendar days of delivery. Manu does not charge a restocking fee. You pay return shipping on ordinary (change-of-mind or customer-initiated) returns; Manu covers return shipping when the return is due to our error, such as a wrong, damaged, or defective item. Contact us for approval before you ship anything back. The full conditions are in the Return & Refund Policy. Questions? Email info@meetmanu.com or call (210) 857-3040. We are in San Antonio, TX 78253.
Frequently asked questions
Can I put ST tires on my truck or SUV?
No. ST tires are made for trailers. They are not designed for the steering and driving loads of a tow vehicle. Use the tire type specified for that vehicle, which is typically P-metric or LT, not ST.
Can I use LT tires on a trailer?
Only with care, and only if that choice is consistent with the trailer manufacturer's guidance. LT and ST tires do not share the same sidewall, load, and speed characteristics. If the placard calls for an ST size and load range, the straightforward path is to replace with that ST size and an equal or higher load range.
Is load range the same as the number of plies?
No. Load range is a capacity rating. The ply rating (for example, "6-ply rating" for load range C) is a strength equivalent, not a count of the physical plies in a modern tire. Use the maximum load in pounds on the sidewall when you check capacity.
How do I measure a 5-lug trailer wheel?
Measure from the center of one lug to the outer edge of the lug two positions away, which approximates the bolt-circle diameter. Confirm with a gauge if you are between common sizes such as 5 on 4.5 and 5 on 4.75. For 4-lug, 6-lug, and 8-lug wheels, measure center to center of opposite lugs.
Can I mix radial and bias-ply trailer tires?
Not on the same axle. Match construction as well as size and load range. If you switch from bias-ply to radial, replace both tires on that axle.
What does a DOT date code like 2324 mean?
The last four digits are the week and year of manufacture. 2324 is the 23rd week of 2024. Trailer tires often need replacement because of age or cracking even when tread remains, so inspect the date code and the sidewall condition together.
