Last updated: August 20, 2026
Boat trailer tires do not usually wear out. They age out. A trailer tire spends most of its life parked in the sun carrying a static load, gets driven hard for a few hundred miles a season, and then fails on the interstate at the worst possible moment, often with plenty of tread still on it. That is a different failure pattern from a car tire, and it calls for a different way of shopping.
This guide explains how to read the size and load range on your existing tire, why special trailer tires exist at all, and what actually matters when you are replacing a pair. No guesswork on numbers: the figures that matter are printed on your own sidewall and published by the tire maker, and this article points you at them rather than inventing them.
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Quick Answer
How do you choose the right boat trailer tires?
- Buy an ST rated trailer tire, not a car or light truck tire, and match the size printed on the sidewall you are replacing.
- Match or exceed the load range letter on the original tire, and check it against your trailer’s capacity plate.
- Radial or bias is a real choice: radials run cooler on long tows, bias tires are common on smaller trailers.
- Replace by age as well as by tread, and always replace and inflate to the figures the tire maker publishes.
Bottom line: copy the size and load range from your sidewall, buy ST rated, and treat age as a wear item.
Why Trailer Tires Are Their Own Category
Trailer tires carry the ST designation, which stands for special trailer, and they are built for a job passenger tires are not asked to do: carry a heavy static load, tolerate side loading in tight turns, and run without any steering or traction duties. The construction is stiffer in the sidewall for that reason.
That stiffness is the point. A trailer tire with a softer passenger car sidewall lets the trailer sway more and heats up faster under sustained load, and both of those are exactly what you do not want behind you at highway speed with a boat on top.
Do Not Fit Car Tires
Passenger and light truck tires appear on trailers regularly and it is a bad substitution. The load ratings, the sidewall construction and the intended duty cycle are all different, and the tire maker’s rating for a P or LT tire is calculated for a driven or steered wheel rather than a trailer axle.
Reading Your Existing Sidewall
Everything you need to buy the replacement is molded into the tire already. You are looking for the size code, the load range letter, the load index and speed symbol, and the four digit date code. Write all four down, or photograph the sidewall, before you start shopping.
The size appears as something like ST205/75R15 on a radial or a two number format such as 20.5X8-10 on the small bias tires common under lighter trailers. Carlisle, for example, sells its Sport Trail line in those small bias sizes for boat trailers and its Radial Trail line in the larger radial sizes, and lists the exact size on each listing.
The Date Code Matters More Than the Tread
The four digit code at the end of the DOT string gives the week and year the tire was built. Because trailer tires typically age out before they wear out, that number is often the deciding factor in whether a tire with plenty of tread should still be on your trailer. Tire manufacturers publish their own service life guidance, so check the maker’s recommendation for the tire you own rather than relying on a rule of thumb.
Load Range and Capacity, Without Guessing
Load range is the letter, such as B, C, D or E, that tells you which strength class the tire falls into, and each combination of size and load range has a maximum load and a corresponding inflation pressure published by the tire maker. Never estimate those two figures. They are printed on the sidewall and listed in the manufacturer’s specification table.
Your trailer has the other half of the equation on its capacity plate or VIN sticker, which states what the trailer is rated to carry. The tires you fit need to cover the loaded weight of boat, engine, fuel and gear across the axles, and matching or exceeding the load range that came on the trailer from the factory is the straightforward way to stay right.
Going Up a Load Range
Some owners step up one load range when the original tires felt marginal. That is a legitimate move if the wheel and rim are rated for the higher inflation pressure the stronger tire requires, which is a question for the wheel manufacturer rather than an assumption. If you are unsure, a trailer shop can confirm it against the wheel’s own rating.
Trailer Tire Picks by Situation
Small Trailers Under Aluminum Boats
Light jon boat and small aluminum trailers usually run small diameter bias tires in sizes like the ones Carlisle lists in its Sport Trail range for boat trailers. Match the exact size code on your sidewall, since these small sizes are not interchangeable by eye.
Long Highway Tows
If you tow several hours to the water, a radial trailer tire is the usual recommendation because radial construction runs cooler over sustained distance than an equivalent bias tire. Match the size and load range, then check the wheel is suitable.
Replacing the Wheel Too
When a rim is pitted from years of saltwater dunking, a tire mounted on a new wheel is often less hassle than remounting onto a corroded one. Buy the wheel by bolt pattern and rim diameter, both of which you can measure or read off the existing wheel.
The Spare You Actually Need
A boat trailer without a mounted spare is a tow truck call waiting to happen, because trailer sizes are not stocked at every gas station. A spare on a matching wheel plus a carrier that mounts to the frame or tongue solves it.
Radial or Bias: What Changes
| Construction | Heat over distance | Typical use | Trade off |
|---|---|---|---|
| Bias ply (ST) | Builds heat faster on long tows | Small trailers, short local runs | Stiff ride, widely available in small sizes |
| Radial (ST) | Runs cooler over sustained miles | Larger trailers, highway distance | Usually costs more, needs a suitable wheel |
Pressure, Storage and What Kills Trailer Tires
Underinflation is the leading cause of trailer tire failure, and it is invisible until the tire is already damaged. Check pressure cold, before towing, against the figure molded on the sidewall for that tire and load range, and check the spare at the same time since it is the one nobody looks at.
The second killer is ultraviolet exposure while parked. A trailer that sits outside all season is cooking its sidewalls, and covers over the tires or simply parking on the shaded side of the building measurably slows that down. Rotating the trailer position occasionally also avoids flat spotting on the same patch of rubber.
Bearings Ride Along With the Tires
While you are under there, look at the hubs. A hot hub after a tow is a bearing problem rather than a tire problem, and a failed bearing takes the wheel with it. Repacking or replacing bearings on a schedule is part of the same job as watching the tires.
Frequently Asked Questions
What size trailer tires do I need? The size molded on the sidewall of the tire you are replacing, matched exactly. If the original tire is missing or you suspect somebody fitted the wrong thing, the trailer’s capacity plate and the wheel diameter are your references, and a trailer shop can confirm the correct size for the axle rating.
Can I put car tires on a boat trailer? No. Trailer tires carry the ST designation and are built with stiffer sidewalls for static load and side loading, and the load ratings for passenger or light truck tires are calculated for a different duty. Fit an ST rated tire.
What does load range mean on a trailer tire? It is a letter such as B, C, D or E that identifies the tire’s strength class, and each size and load range combination has a maximum load and inflation pressure published by the tire maker and molded on the sidewall. Match or exceed what the trailer came with.
How often should boat trailer tires be replaced? Age is usually the trigger rather than tread depth, since trailer tires spend most of their life parked. Check the four digit date code on the sidewall and follow the tire manufacturer’s own service life guidance for that product.
Radial or bias for a boat trailer? Radials run cooler over long distances and are the usual pick if you tow several hours to the water. Bias tires remain common and perfectly serviceable on smaller trailers doing short local runs.
Why do my trailer tires wear on the inside edge? Uneven wear on a trailer usually points at alignment, a bent axle, or chronic underinflation rather than the tire itself. Worth having a trailer shop look at the axle before you spend on another set.
The Bottom Line
Copy the size and load range off your own sidewall, buy an ST rated trailer tire from a maker that publishes clear specifications, and check pressure cold before every tow. Treat the date code as seriously as the tread, keep a mounted spare on the trailer, and watch the hubs while you are at it. Lights are the other thing that quietly fails between seasons: our guide to boat trailer lights covers submersible LED kits that survive the ramp.