Understanding Tyre Load Ratings for Trucks and Trailers

Every commercial tyre carries a code on its sidewall that tells you exactly what it can and cannot do. Most fleet managers and drivers walk past this information every day without reading it. That is a problem — because fitting a tyre with the wrong load rating is not just an expensive mistake, it is a safety and legal issue.

This guide explains what the markings on a commercial tyre sidewall mean, how to use them to specify the right tyre for your application, and what happens when you get it wrong.

The most prominent marking on a commercial tyre sidewall is the size designation. For a typical truck tyre, it looks something like this:

315/80 R22.5

Breaking this down:

• 315 — the tyre width in millimetres, measured from sidewall to sidewall when mounted on the specified rim width
• 80 — the aspect ratio, expressed as a percentage of the tyre width. In this case, the sidewall height is 80% of 315mm = 252mm
• R — radial construction (virtually all modern commercial tyres are radial)
• 22.5 — the rim diameter in inches

This tells you the physical dimensions of the tyre. It does not tell you the load capacity — that comes from the load index.

The load index is a numerical code that indicates the maximum load a single tyre can carry at its rated inflation pressure. It appears after the size designation, typically alongside the speed rating.

For example: 315/80 R22.5 156/150 L

Here, 156/150 is the load index — 156 for single fitment, 150 for dual fitment. The difference exists because tyres in a dual configuration run slightly hotter and are subject to different stress patterns than single tyres.

These values are at the tyre's rated inflation pressure. Running at lower pressure reduces the load capacity — which is why inflation pressure management is so critical.

The speed rating appears as a letter after the load index. For commercial tyres, the most common ratings are:

• J — 100 km/h
• K — 110 km/h
• L — 120 km/h
• M — 130 km/h

South Africa's national speed limit for heavy vehicles is 80 km/h on national roads, so most commercial tyres are rated well above the legal maximum. However, the speed rating also affects the tyre's heat generation characteristics — a tyre operated consistently at or near its speed rating runs hotter and wears faster.

To determine whether a tyre is suitable for a specific axle position, you need to know the axle load. This is found on the vehicle's registration certificate and should not exceed the legal axle load limits.

For single tyres: The tyre's load index (single) must equal or exceed the axle load.

For dual tyres: The tyre's load index (dual) multiplied by two must equal or exceed the axle load.

A drive axle has a rated load of 7,500 kg and is fitted with dual tyres. Each tyre must carry a maximum of 3,750 kg. You need a tyre with a dual load index of at least 154 (3,750 kg).

A tyre marked 154/150 would be marginal — the dual rating of 150 (3,350 kg) is insufficient. You would need a tyre with a dual rating of at least 154.

Load capacity is not a fixed value — it varies with inflation pressure. Tyre manufacturers publish load/inflation tables that show the maximum load at various inflation pressures. Running a tyre at lower than its rated pressure reduces its load capacity, which means a tyre that is correctly specified at full inflation may be overloaded if the pressure drops.

This is why under-inflation is so dangerous on loaded commercial vehicles. A tyre running at 80% of its rated pressure may have a load capacity 10% to 15% lower than its rated maximum. On a vehicle operating at or near its GVM, this can push the tyre into an overloaded condition — with all the associated risks of heat build-up, casing damage, and eventual failure.

Older tyres and some current bias-ply tyres carry a PR (ply rating) designation rather than a load index. The ply rating is a historical measure of tyre strength that originally referred to the number of fabric plies in the tyre casing. Modern tyres use fewer, stronger plies, so the ply rating is now a comparative strength rating rather than an actual ply count.

For modern radial commercial tyres, the load index system has largely replaced the ply rating. However, you may still encounter PR ratings on older vehicles or replacement tyres for older equipment.

Fitting a tyre with an insufficient load rating is a legal offence in South Africa. The National Road Traffic Act requires that tyres fitted to a vehicle must be appropriate for the vehicle's design and the loads it carries. A vehicle found to be operating with underrated tyres can be impounded, and the operator faces fines and potential liability for any accidents caused by tyre failure.

More importantly, insurance claims arising from accidents caused by tyre failure may be repudiated if the investigation reveals that the tyres were not correctly specified for the vehicle's load.

The correct approach to tyre specification for a commercial vehicle is:

1. Obtain the vehicle's axle load ratings from the registration certificate
2. Calculate the required load index for each axle position (single or dual)
3. Select a tyre with a load index that meets or exceeds this requirement
4. Confirm that the inflation pressure required for the load is within the tyre's rated range
5. Document the specification so that replacement tyres are always correctly matched

If you are unsure about the correct tyre specification for any vehicle in your fleet, contact Apex Tyre Solutions on +27 079 294 4277. Our team can help you verify your current fitments and identify any positions where the specification needs to be corrected.