How to Know Correct Tire Inflation Pressure for 21-Foot Trailer  

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Expert Reply:

Good question.

Trailer length and weight do not tell you what the correct tire pressure is. That will be found on the tires themselves, right on the tire's outer sidewall. With trailer tires the cold tire pressure should ALWAYS be the maximum psi pressure noted on the tire. It is never appropriate to under- or over-inflate a trailer tire for any reason.

Trailer tires can deliver their rated weight-carrying capacity only when fully inflated to their maximum cold psi pressure. Under- or over-inflation will lead to excess heat in the tire which in turn will often lead to a blow out. At the very least incorrect inflation pressure can mean uneven tread wear. Over-inflated tires will tend to wear faster in the middle of the tread; under-inflated tires will tend to wear faster on both the inside and outside edges of the tread surface.

Excess heat in a trailer tire can come not only from incorrect inflation pressure but from overloading the tire beyond its weight rating, or from driving it faster than it is made to go. Trailer tires vary in their speed ratings from the nominal 65-mph up to speeds as high as 81-mph, what is considered an M speed rating. Operating your tires at correct pressure and within their weight and speed limits will ensure the longest possible lifespan. Please note that regardless of mileage or tread condition trailer tires should be replaced no less often than every 6 years.

Your trailer's main information/VIN sticker may indicate the axle weight rating and/or a GVWR, the maximum the loaded trailer can weigh under any circumstances. Your tire choice should be such that the total trailer weight is below the total load capacity of the tires. In other words, divide the trailer GVWR by the number of tires on it (2 for single axle, 4 for tandem axle, etc) and be sure to select tires in the Load Range rating that provides a healthy safety margin. All the tires together should be able to handle more weight than the trailer GVWR.

Two helpful articles on trailer tires are linked for you along with our main page for all radial and bias ply trailer tires we offer.

expert reply by: Adam R

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