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11 Things To Know About Boat Trailer Tires

Tires aren't exciting, until something goes wrong. Then, tires become the topic of the day. Here are some tire basics so that your tires will always be, well, tiresome.

Two vehicles on the highway side-by-side towing powerboats

Trailer Made

A tire's sidewall is where you learn everything: dimensions, capacities, age, and most important, purpose. A tire made for a boat trailer is going to have "ST" on the sidewall, or the words "Trailer Use Only." Unlike tires on your car, trailer tires have strengthened sidewalls to handle the weight of a boat, especially when rounding corners.

Trailer tires are either bias ply or radial. Bias-ply sidewalls are stiffer, less expensive than radials, and are preferred if the trailer isn't used for long trips. If you take long trips, then radials are a better choice because there's reduced heat buildup as compared to bias ply, greater load capacity, and less road noise. Use all bias ply or all radials; never mix them.

Boat trailer radial tire sidewall markings close-up


Check inflation prior to going on the road when the tires are cold. The tire pressure measured in pounds per square inch (psi) will increase as the tire heats up when used. BoatUS TRAILER ASSIST service providers say tires are the main reason they're called to help members experiencing trouble on the road. Under inflation is the cause of most tire trouble because temperatures increase when the tire pressure is too low. So, check inflation prior to going on the road. It's marked on your tire. Remember, boat trailer tires typically need to be inflated to higher psi than tow vehicle tires. And be sure to also check the psi of the spare tire.

Load Range

Every tire has a load range, and trailer tires are no different. Marked on the sidewall, the load range runs from the lightest weight the tire can carry (Load Range B) to the heaviest (Load Range E). Load range is a measure of an individual tire's maximum capacity to carry a boat and trailer. Most boat trailer tires have a load range of B, C, or D. If a tire has load range C, it can carry 1,820 pounds. If it's on a single-axle trailer, this means both tires can carry a total of 3,640 pounds, which includes the weight of the trailer, the boat, the engine, fuel, and anything else inside the boat. Single-axle trailers can carry 100 percent of the load rating. Double-axle trailers require the load be reduced by 12 percent. As load range increases, psi increases.

Load range markings on the sidewall of a trailer tire

Tire Wear

Your tires will tell you when something's wrong and usually give plenty of hints before becoming the topic of conversation. Tire pressure that's too high causes center wear on the tire while low tire pressure causes wear on the outside edges.


Inspect the sidewalls for spiderweb cracks, which is evidence of dry rot (and imminent failure).


A Lincoln penny is a good measure for tread depth on both car and boat trailer tires. Place the penny upside down on the tread. If you can see the top of the president's head, the tread is worn and it's time to start looking for new tires.

Copper penny stuck in tire to test the depth of the tire tread

Valve Stem

Often overlooked, the valve stem may be the cause of continued low psi. Press your finger against one side of the stem and listen for any loss of air.

Balance And Rotate Tires?

The answers are "yes" and "maybe." Like tires on your car or tow vehicle, trailer tires that are balanced ensure proper tire wear and reduce vibration. Single axle boat-trailer tires usually don't need to be rotated. A tandem axle trailer rotation isn't necessary unless you notice increased wear that's occurring faster than normal. In that case, rotate the tires from front to back, preferably in an "X" configuration.

All About Boat Trailer Tires

If The Trailer Sits Outside

Trailer tires face a pair of threats outside:

  • Sun: Prolonged exposure to the sun's UV rays will deteriorate trailer tires, causing spiderweb cracks. This can be easily resolved by putting tire covers over the tires if the trailer is going to be parked for an extended period of time.
  • Moisture: The worst thing you can do to a trailer tire is to park it on grass for a long period of time, where moisture is going to slowly get into the tire's protective surface. This can be solved by either removing the tires during the winter (also deters theft) or positioning the trailer tires on concrete or plywood.

Motion Is Good

A parked trailer means the tires are bearing the weight in the same position on the tire, for a long period of time. This can result in "flat spots" on a tire. If the trailer is parked for a while, check the tire pressure from time to time to ensure the tires are properly inflated.


Most trailer tires have a maximum speed of 65 mph. However, if speeds from 66 to 75 mph are used, the tire inflation pressure can be increased by 10 psi without any increase in load. However, the Tire and Rim Association, which sets standards for tires, suggests decreasing the load capacity of the trailer by 10 percent if you are running at these higher speeds.

The PSI — A Tire Pressure Monitoring System

After all the noise we've made about the importance of checking tire pressure, it can be done without getting out of your truck with a tire gauge. A number of monitoring systems are available for recreational vehicle owners but here's one designed with a trailer boat owner in mind. The 507RV made by Truck System Technologies is part of a larger system already in use on semitrucks throughout the country (Freightliner, MHC Kenworth, to name a few). It's simple to set up: A monitor is screwed into the tire stem and a screen is plugged into the tow vehicle cigarette lighter and, once programmed, will provide the psi of each tire as well as the temperature inside the tire (the latter being important to warn that a bearing or brake rotor is going bad). It works when trailers are dunked in fresh water or saltwater, too.

Tire pressure digital readout on the screen

How Old R U?

Your tire's age (whether for a boat trailer, truck, or passenger car) has a DOT (Department of Transportation) time stamp that provides the tire's date of manufacture. The last four digits tell you everything you need to know. In this example, the tire was manufactured in the second week of 2008.

Department of Transportation tire time stamp with labels explaining the meaning of each number or letter

What Does It Mean?

Sidewall of a trailer tire and its markings

ST: This tire is designed for a trailer.

205: Width of the tire in millimeters.

75: The ratio of height to width. In this example, the tire's height is 75 percent of its width. The smaller the aspect ratio, the wider the tire in relation to height.

D: This indicates a bias ply tire."D" stands for diagonal. "R" would mean this is a radial tire.

14: Wheel diameter: This trailer has 14" rims.

Tire Wear Chart

Tire wear illustration

Did You Know?

  • Most boat trailer tires have a maximum speed of 65 mph.
  • 75 percent of drivers wash their cars at least once a month while only 1 out of 7 (14 percent) correctly check tire pressure, according to Bridgestone.
  • Every 10-degree decrease in air temperature leads to 1 psi of air loss. If you’re traveling somewhere that the morning temperature varies greatly from the afternoon temperature, check your air pressure first thing in the morning and again as the temperature increases.

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Pat Piper

Contributor, BoatUS Magazine

Pat Piper has written for many magazines including BoatUS Magazine, Chesapeake Bay Magazine, and Diabetes Health and has written/ghostwritten eight books. He's currently an executive editor for Westwood One Radio.