Load index and speed symbol explained

The load index is a code, not a weight. It points at a row in a standard table which gives the maximum load a tyre may carry at a stated inflation pressure and at the speed given by its speed symbol. Load index 156 means 4,000 kg; 150 means 3,350 kg. The two markings only make sense together.

On a commercial tyre the service description reads 156/150L. Two indices, one symbol. Below is what each part governs, and how to check the required specification.

Moulding a kilogram figure onto a sidewall would be simpler, and the sidewall does in fact carry one. The index exists alongside it because it is a shared, standardised scale: it lets a regulator, a vehicle manufacturer and a fitter refer to the same capacity in two or three characters, in any language, on any tyre from a moped to an articulated dump truck.

The scale is not linear and the steps are not even. Each step upwards is roughly three per cent more load, but the actual increments are set by the table, not by a formula. From 155 to 160 the values run 3,875, 4,000, 4,125, 4,250, 4,375, 4,500 kg — 125 kg a step. Higher up they widen. That is why you look the number up rather than calculating it, and why “a couple of indices higher” is not a safe way to talk about a specification.

Commercial tyres carry two load indices because they may be fitted singly or in a twin. 156/150 means 4,000 kg per tyre in single fitment and 3,350 kg per tyre in dual — about sixteen per cent less.

The reduction is not caution for its own sake. Two tyres bolted either side of a wheel centre never carry an equal share of the load. Road camber puts more weight on one; a difference in pressure between the pair transfers load to the harder tyre; a difference in diameter through wear or a mismatched replacement does the same; kerbing and yard manoeuvring load the outer tyre in ways the inner never sees. The dual index is the standard’s allowance for that inequality.

For correct specification, on a twin fitment, use the second number. A trailer axle plated at 9,000 kg needs 4,500 kg per axle end, which across a twin is 2,250 kg per tyre — comfortably inside a 150 dual index. Do the same sum for a 4-tonne front axle in single fitment and the first number is the one that has to cover it.

The speed symbol is the maximum speed at which the tyre may carry the load given by its index. It is not a top speed for the tyre in isolation, and operating limits must still be observed.

The two ratings trade against each other. Below the rated speed a commercial tyre can generally carry more than its index suggests — which is why site, crane and low-speed industrial fitments have published uplift tables allowing substantially higher loads at 30 km/h or below. Above the rated speed, capacity falls away and heat build-up becomes the limiting factor rather than structural strength. That is why many truck tyres carry a second, bracketed service description such as 156/150L (154/150M): a lower load permitted at a higher speed.

Those variation tables belong to the manufacturer and to the standards bodies. Do not interpolate your own. Where a vehicle spends its life at low speed under heavy load — port tractors, shunters, heavy plant carriers — ask for the specific low-speed load table for that pattern rather than assuming the sidewall figure is the ceiling.

Two symbols catch people out. H sits out of alphabetical order at 210 km/h, above U, for historical reasons — it is a car rating you will meet on van and light commercial fitments. And the A1 to A8 range covers 5 to 40 km/h, which is where most materials handling and industrial tyres live. A forklift tyre marked A5 is rated for 25 km/h, and a machine geared beyond that is outside the tyre’s approval.

A load index is only valid at the reference inflation pressure for that tyre. The air carries the load; the casing contains the air. Drop the pressure and the tyre’s real capacity drops with it, whatever the sidewall says.

Under-inflation does not fail the tyre by squashing it. It fails it by increasing sidewall flex, which generates heat in the casing and belt edges, which degrades the rubber-to-steel bond. A tyre run at twenty per cent below its correct pressure for a few thousand kilometres may still look serviceable and may already be finished as a retread casing. That is the single most expensive avoidable cost in a truck fleet, and it is why casing value belongs in the retread or replace calculation.

Set pressures from the manufacturer’s load and pressure table against the actual axle load, not from the maximum on the sidewall and not from a previous pressure setting. In agricultural work the same principle runs the other way — see agricultural tyre pressure and IF and VF tyres, where the whole point of the technology is carrying the same load at lower pressure.

Legally, a tyre whose load or speed rating is below what the vehicle requires is a defect. Roadworthiness standards treat it as such, and it is the kind of finding that attracts a prohibition at the roadside rather than a note in the file. The vehicle manufacturer’s plated axle weights are the reference, not the weight the operator says the vehicle usually carries. Where a tyre is fitted below specification and something happens, the position on insurance and on operator licence repute is not somewhere any dealer wants their invoice to appear.

Practically, the failure mode is heat. An under-rated tyre run at or near its limit runs hot, the casing degrades from the inside, and it fails as a rapid deflation or a belt separation rather than a slow puncture. On a steer axle that is a serious incident. On a trailer twin it usually destroys the partner tyre as well, which is why a single under-specified tyre often turns into a four-figure roadside bill.

When considering a substitution, a customer wants a size you have, in a lower index, at a better price. The correct answer is the index the axle needs. Check it against the plated weight with the axle load calculator before quoting, not after fitting.