Load index lookup

The load index is the two or three digit number immediately before the speed symbol. In 315/80R22.5 156/150L it is the 156, and the 150 next to it is the second index.

Where a tyre carries two, they are not alternatives. The first is the load per tyre in single fitment, the second is the load per tyre in dual fitment. The dual figure is always lower, because twinned tyres never share load evenly — road camber, kerbing, pressure differences between the pair and any mismatch in diameter all push load onto one of the two.

Enter whichever index applies to the position you are specifying, then set the quantity to the number of tyres you want totalled: two for a single-fitment axle, four for a twin axle, six for a three-axle trailer running singles, and so on. The calculator returns the load per tyre in kilograms and pounds, and the total in kilograms and tonnes.

Take a twin-fitment drive axle on a tractor unit running 315/80R22.5 156/150L, plated at 11,500 kg.

Enter 156 with a quantity of 4 and you get 4,000 kg per tyre, 8,818 lb, and 16,000 kg — 16.00 tonnes — across the four. That is the single-fitment figure, and on a twin axle it is the wrong one to use.

Enter 150 instead, the dual index, and you get 3,350 kg per tyre and 13,400 kg across the axle. Against a plated 11,500 kg that leaves 1,900 kg of headroom, which is a comfortable specification. Had the answer come out under 11,500 kg, the tyre would have been wrong for the axle regardless of what the first index said.

The same arithmetic run the other way is what the axle load calculator does: give it the axle weight and it tells you the minimum index rather than making you work back from the tyre.

Every figure here comes from the ISO and ETRTO load index table, covering index 50 to 200. Nothing is interpolated, curve-fitted or rounded on the way in. Load index 116 is 1,250 kg, load index 164 is 5,000 kg, and they are those numbers because the table says so.

The only arithmetic applied is the conversion to pounds at 2.20462 lb per kg, rounded to the nearest pound, and the multiplication by the quantity you enter.

What the table assumes is more important than what the calculator does. A load index is the maximum load a tyre may carry at the inflation pressure marked on its sidewall, at or below the speed its speed symbol allows, in normal service. Take pressure away and the capacity goes with it — an under-inflated tyre does not carry its index, whatever is written on the side. Pressure, not the marking, is the thing that actually carries the load.

The index tells you what the tyre can carry. It says nothing about what the axle is rated for, what the vehicle is plated at, or what the rim can take. All four have to be checked, and the lowest of them governs.

Several load systems sit outside this table entirely:

• Agricultural cyclic field work. Tractor and implement tyres are permitted substantial load bonuses at low speed for field operation, and are derated for road speed. Those come from the manufacturer’s load and pressure table, not from the index alone.
• IF and VF constructions. High-flexion tyres carry more at the same pressure, or the same load at lower pressure. The index on the sidewall already reflects that, but the pressures behind it do not match a standard tyre of the same index.
• Earthmover and quarry tyres. Load is governed by TKPH and TMPH, star rating and the duty cycle of the machine, not by an ISO index.
• Solid and cushion industrial tyres. These are rated by load per tyre at a stated speed and duty, with no index at all.

One more: a tyre may carry a second service description in brackets for a higher speed at a lower load. This calculator takes one index at a time, so check both.

The range that covers commercial work, from light van through to heavy earthmover. Index 100 is 800 kg; index 199 is 13,600 kg. The step between adjacent indices is around 2 to 3%, which is why moving up one index is a small change and moving up five is a different tyre.

Older commercial, agricultural and industrial sizes are marked with a ply rating such as 16PR, or a load range letter such as G, instead of an index. The two systems are equivalent to each other but neither converts directly to a load index, because the load a given casing strength supports depends on the size of the tyre. A 16PR light truck tyre and a 16PR agricultural tyre carry very different weights.