Trailer Tire Load Calculator
The load figure on a trailer tyre sidewall comes with a condition most people miss: it is valid only at the maximum cold pressure stamped beside it. Run 50 psi in a tyre rated at 65 and you no longer have the capacity printed on it — and the tyre runs hotter, which is what actually destroys most trailer tyres. This calculator works out the load on each tyre, checks the reserve, and flags the pressure.
- Within tyre capacity
- 1,540 lb per tyre Against 2,150 lb stamped capacity — 40% reserve
- Weight on the axles
- 6,160 lb 7,000 lb trailer less 840 lb tongue weight, which rides on the tow vehicle
- Total tyre capacity
- 8,600 lb 4 tyres × 2,150 lb — only valid at 65 psi cold
- Pressure
- 65 psi — at rating At the rated cold pressure, so the stamped capacity applies. Check when cold — pressure rises several psi once rolling.
- Recommended reserve
- 10–20% Trailer axles rarely share load evenly, and a corner load or uneven pavement can put substantially more on one tyre than the average shown here.
- Speed rating
- Check the sidewall Most ST trailer tyres are rated for 65 mph unless marked otherwise. Sustained travel above the rating builds heat the tyre was never designed to shed, and heat is what causes the tread separations trailers are known for.
Heat is what kills trailer tyres
Trailer tyres rarely fail from wear. They fail from heat, and almost everything that causes heat is under the owner’s control.
A tyre flexes as it rolls. That flexing generates heat internally, in the rubber and in the belt package, and the tyre sheds it through its surface as it turns. When heat is generated faster than it can be shed, the temperature climbs until the bond between the tread and the belts weakens and the tread separates — usually at speed, usually with no warning.
Underinflation is the largest single cause because it increases flex directly. A tyre at 50 psi where 65 is specified has a taller, softer sidewall working harder every revolution. Overloading does the same thing for the same reason. Speed compounds both, because flex cycles arrive faster and there is less time to cool between them.
Age is the quiet one. The rubber compounds oxidise over time whether the trailer moves or not, and a tyre stored outdoors in sun degrades faster still. Five to seven years is the widely used replacement interval regardless of tread depth, and trailer tyres frequently age out long before they wear out.
Reading the sidewall
Everything you need is moulded into the tyre, though it takes knowing where to look.
The size begins with ST for a special trailer tyre — as opposed to P for passenger or LT for light truck. ST tyres carry more load for their size with stiffer sidewalls, and have shorter speed ratings and service lives in exchange.
The load figure appears as a maximum load in pounds and kilograms, immediately followed by the cold inflation pressure it applies at. Those two numbers are a pair. Quoting the load without the pressure is meaningless, and this is the detail most people miss.
The load range letter — C, D, E — indicates the construction and corresponds to a maximum pressure: roughly 50 psi for C, 65 for D and 80 for E. Moving up a load range is how you get more capacity in the same size.
The DOT code ends in four digits giving the week and year of manufacture. 3223 means the thirty-second week of 2023. This is how you check age, and it is worth checking on a new trailer — tyres can sit in inventory for a year or more before they are fitted.
Why reserve capacity matters more on a trailer
A car distributes load between four tyres on a suspension designed to keep them evenly planted. A trailer does neither well.
Tandem axles share load through the suspension, and that sharing is imperfect even on level ground. Crossing a driveway apron, parking on a slope or hitting a pothole momentarily transfers a large fraction of one axle’s load onto the other. Equaliser bushings wear and the sharing gets worse with age.
Cargo is rarely centred. A trailer loaded with weight to one side puts more on the tyres on that side permanently, and nothing in the suspension corrects for it.
This is why ten to twenty percent reserve is standard advice rather than an abundance of caution. The calculated average load per tyre is not what any individual tyre sees at any given moment, and the tyre that fails is the one that saw the most.
What this is based on
- Tyre sidewall maximum load and corresponding cold inflation pressure
- Industry practice of 10–20% load reserve on trailer axles
- Typical ST trailer tyre speed rating of 65 mph unless otherwise marked
An estimate assuming even load sharing between tyres, which real trailers rarely achieve. Follow the tyre manufacturer’s load and inflation tables, and the trailer manufacturer’s placard, over any calculated figure.
Frequently asked questions
Why does pressure change tyre capacity?
Because air carries the load, not rubber. The stamped capacity is measured at the stamped pressure; below it the sidewall flexes more, the contact patch distorts and the tyre builds heat. Load capacity and inflation are a pair, and quoting one without the other is meaningless.
Should tongue weight count against the trailer tyres?
No. Tongue weight rests on the hitch and therefore on the tow vehicle, so it comes off the trailer axle load. It still has to be counted against the tow vehicle’s payload and rear axle, just not here.
How much reserve should I leave?
Ten to twenty percent. Trailer axles do not share load evenly, ground is not flat and cargo is rarely centred, so any individual tyre can see noticeably more than the calculated average. Running at exactly 100% of rating leaves nothing for that variation.
Why are ST tyres limited to 65 mph?
They are built with stiffer sidewalls for load carrying rather than for heat dissipation at speed, and trailers have no suspension damping comparable to a car. Above the rating heat accumulates faster than the tyre can shed it, which is the mechanism behind most trailer tread separations.
Are trailer tyres different from car tyres?
Yes, and the two are not interchangeable. ST tyres have stronger sidewalls to resist sway and carry more load for their size, but lower speed ratings and shorter service life. Trailer tyres commonly age out before they wear out — five years is a widely used replacement interval regardless of tread.