Trailer Tire Speed Rating Chart and How to Read the Codes

October 21, 2026 · 808 words

What a Trailer Tire Speed Rating Means

A trailer tire speed rating is the maximum speed at which the tire is certified to carry its stated load. The rating appears as a letter code in the tire's service description, usually after the load index. For example, a tire marked ST205/75R15 101M has a load index of 101 and a speed rating of M, which corresponds to 81 mph. The speed rating applies only when the tire is properly inflated, not overloaded, and in good condition. Exceeding the rated speed, even briefly, can cause heat to build up faster than the tire can dissipate it, which is a leading cause of trailer tire failure.

Trailer Tire Speed Rating Chart

The letters used for trailer tires are a subset of the standard passenger tire speed rating system. Most ST, or Special Trailer, tires are rated from J through N. Higher letters indicate a higher maximum certified speed. The common codes for trailer tires are listed below.

Trailer tires rated above N exist, but they are less common on ST tires. In practice, most bias-ply and many radial trailer tires carry an L or M rating. An N rating is often found on higher-speed radial trailer tires used for enclosed cargo or car haulers. The chart reflects the standardized maximum speed for each letter under laboratory test conditions, not a recommended cruising speed for every load and road surface.

ST Tire Speed Ratings and the 65 mph Tradition

For decades, most ST tires carried a rating between J and L, and the trailer industry widely advised keeping speeds at or below 65 mph. That guidance persists even though many modern radial ST tires carry M or N ratings. The lower historical limit was tied to older bias-ply construction and heat management, not to a universal legal speed limit. Today, a tire with an M rating is certified for 81 mph, but manufacturers may still recommend a lower maximum speed for extended highway use, especially in hot weather or at maximum load. The speed rating should be read as a certification ceiling, not an all-conditions target.

Trailer Tire Speed Rating Location

The speed rating is molded into the tire sidewall as part of the service description. It follows the load index and is separated from it by no space or punctuation. For instance, a marking of 205/75D15 107L shows a load index of 107 and a speed rating of L. The rating may also appear in the tire's full DOT serial information, but the service description is the intended place to read it. If the sidewall is worn or heavily weathered, the raised characters can be difficult to see, and using a light at a low angle can help.

Load Index and Speed Rating Work Together

The speed rating is only valid at the load indicated by the load index. A tire marked 1765 lbs at 81 mph in an M rating is not also certified for 1765 lbs at any higher speed. Conversely, reducing the load does not automatically raise the certified speed. Trailer tire load and speed ratings are paired values from a standardized table. When selecting replacement tires, matching both the load index and the speed rating, or exceeding them, preserves the trailer's original certified capabilities.

Speed Rating for Smaller Trailer Tires

Small tires such as a 4.80-12 are often fitted to utility trailers, boat trailers, and small cargo trailers. A 4.80-12 bias-ply tire commonly carries a J or K rating, meaning 62 or 68 mph maximum. Radial versions of similar size often carry an L or M rating. The exact rating depends on the specific tire model and manufacturer, but the smaller diameter and higher rotational speed at a given road speed make the lower ratings common. A J-rated tire on a trailer towed consistently at 70 mph is being operated beyond its certified limit, even if it appears to be in good condition.

Using the Chart When Replacing Tires

Replacement trailer tires should meet or exceed the load index and speed rating of the original tires. Installing a tire with a lower speed rating than the original reduces the certified top speed of that axle. Installing one with a higher rating is acceptable, provided the load index, dimensions, and inflation pressure requirements are compatible with the wheel and trailer. Mixing tires with very different speed ratings on the same axle is not recommended because it can create uneven handling characteristics and unequal heat buildup.

Practical Takeaways

Read the service description on the sidewall; the letter after the load index is the speed rating. Use the chart to convert the letter to a maximum certified speed; J is 62 mph, K is 68 mph, L is 75 mph, M is 81 mph, and N is 87 mph. Treat the rating as a hard ceiling, not a recommended cruising speed. Match or exceed the original load index and speed rating when replacing trailer tires.