Trailer Tire Load Calculator

Special Trailer tyres are a distinct category, and the most consequential thing about them is not widely known: their load ratings are calculated on the assumption of a maximum operating speed of 65 mph.

This is not a footnote. The Tire and Rim Association load formula for ST-designated tyres builds the 65 mph assumption in, which is part of why ST tyres carry load ratings 10 to 20% higher than a light truck tyre of the same size. The extra capacity is bought with a speed restriction.

The guidance above 65 mph is specific. Between 66 and 75 mph, cold inflation must be raised 10 psi above the placard pressure and the full load rating still applies. Between 76 and 87 mph, the pressure increase still applies and the load must additionally be reduced by 10%. Above 87 mph there is no rating at all.

The problem is that most trailer placards already specify the tyre’s maximum sidewall pressure, because unlike car tyres, trailer tyres are sized to their load and run at maximum. When the placard says 65 psi and Load Range D tops out at 65 psi, there is no room to add ten. The only compliant responses are a slower speed or a higher load range.

This puts a large number of trailers on the road in a position their owners are unaware of: within their load rating, above their speed rating, and unable to meet the pressure requirement that would make the speed acceptable.

Tyre failure is the most common serious trailer incident on the road, and the two dominant causes are under-inflation and age. Tread depth is almost never the reason.

Under-inflation kills tyres through heat. A soft tyre flexes further in the sidewall on every revolution, that flexing generates heat, and heat degrades the bond between the rubber and the steel belts. The damage accumulates internally where no external inspection will find it, which is why a tyre can look perfect on the morning it fails. Running a trailer tyre ten psi low for a few hundred highway miles is enough to do real harm.

Age kills them through oxidation. A trailer typically stands still for most of the year, often outdoors in sunlight, and the rubber compound oxidises and cracks whether the tyre is rolling or not. The standard guidance is replacement at five to six years from the date code irrespective of tread depth, and most trailer tyres are discarded with the great majority of their tread remaining.

The four-digit DOT date code is the number to know. Stamped on the sidewall, "3223" means the 32nd week of 2023. It is the tyre’s manufacture date, not its purchase date — a tyre can sit in a warehouse for a year or two before being fitted, which is worth checking when buying.

Running trailer tyres right at their rating is legal and unwise, and the usual recommendation is to keep at least 20% in reserve. There are three reasons, and they compound.

First, load is never shared evenly. A trailer sitting on a crowned road, a load that is heavier on one side, an axle slightly out of alignment — each puts more weight through some tyres than others. The nominal even split is an average, and the tyre carrying the most is the one that decides the outcome.

Second, a single failure transfers everything. When one tyre on a tandem axle fails, its share does not disappear; it lands on the tyre beside it, often instantaneously and at speed. A tyre already at 95% of rating is not going to survive that transfer, which is how one blowout becomes two and a modest problem becomes a serious one.

Third, the load figure you calculated is optimistic. It came from a static weight on level ground. Road inputs, braking, cornering and any sway event all produce dynamic loads well above the static number.

Twenty percent absorbs all three. It is why moving up a load range is frequently the right purchase even when the current tyres are technically adequate.

Three tyre types get fitted to trailers, and they are not interchangeable in the way their sizes suggest.

Trailer tyres respond to a short list of unglamorous habits, and they are the difference between tyres that reach their age limit and tyres that do not.

Check pressures cold, before the first drive of the day, every travel day. Cold means before the trailer has moved — a tyre warmed by ten miles of highway reads several psi high, and setting it to placard while warm leaves it under-inflated once it cools. Trailer tyres run at maximum sidewall pressure, so there is no judgement involved: set it to the placard.

Check the inner tyres on any dual-wheel axle. They are hard to reach, easy to skip, and the failure mode is severe — a deflated inner tyre transfers its full load to the outer one and destroys it within a few miles, usually without the driver noticing anything.

Look at the sidewalls for fine cracking at the base of the tread blocks and around the bead. That is what age failure looks like before it becomes a blowout.

Cover the tyres when parked for long periods, or park on boards rather than directly on hot tarmac. Ultraviolet light and heat are what age the rubber, and a trailer that spends eleven months a year sitting outside is ageing its tyres the whole time.

And carry a working spare that is the same age question as the others. A five-year-old spare that has never been fitted is a five-year-old tyre.