Sidewall decoder

Enter the size exactly as it is moulded on the tyre. Both conventions work. Metric sizes carry an aspect ratio between the width and the construction letter — 315/80R22.5, 385/65R22.5, 520/85R42. Imperial sizes do not — 11R22.5, 12.4R28, 20.5R25, 8.25-15.

The letter between the width and the rim diameter tells the decoder how the casing is built: R for radial, D or a plain dash for crossply. Case and spacing do not matter, so 315/80r22.5 and 315/80 R 22.5 both work.

Add the service description if the tyre carries one — 156/150L, 157A8, 148/145K — and both load indices and the speed symbol are looked up as well. A ply rating written as 16PR is picked up and converted to its load range letter.

What comes back: section width in millimetres, aspect ratio with the sidewall height worked out, construction, rim diameter in inches and millimetres, overall diameter, the load index or indices converted to kilograms per tyre, and the speed symbol converted to a maximum speed.

Put in 315/80R22.5 156/150L, as an example, and the decoder returns:

• Section width 315 mm
• Aspect ratio 80% — sidewall height 252 mm
• Construction radial
• Rim diameter 22.5 in (572 mm)
• Overall diameter 1076 mm (42.3 in)
• Load index 156 — 4,000 kg per tyre, single fitment
• Load index 150 — 3,350 kg per tyre in dual fitment
• Speed symbol L — 120 km/h (75 mph) maximum

Use the load index for the correct fitment position. On a twin-fitment drive or trailer axle it is the second figure that applies, because twins never share load perfectly. The gap here is 650 kg per tyre — 2,600 kg across a twin axle, which is enough to change whether a specification works.

Try an imperial size next, say 8.25-15. The decoder returns 8.25 in (210 mm nominal) section width, crossply construction, a 15 in rim, and an overall diameter of 767 mm — with a note explaining that the aspect ratio was assumed, because the sidewall does not state one.

The decoder combines table lookups with nominal dimensional calculations.

The load index and speed symbol are looked up, and they are exact. They come from the ISO and ETRTO tables that every manufacturer publishes against. For example, load index 156 corresponds to 4,000 kg in the reference table.

The overall diameter is calculated, and it is nominal. The decoder does the standard arithmetic — rim diameter plus twice the sidewall height, where sidewall height is section width multiplied by aspect ratio. That is geometry derived from the marking, not a measured dimension. The figure a manufacturer publishes for a specific pattern will normally sit a few millimetres either side of it, because moulded dimensions depend on the mould, the rim the tyre is fitted to and the tread depth of that pattern.

Section width is nominal too. It is the design width on the design rim. Fit the same tyre on a rim at the wide end of its approved range and the section grows; fit it narrow and it pinches in.

Imperial sizes carry no aspect ratio at all, so the decoder applies the industry nominal of 92% and says so in the output. Real aspect ratios in the older imperial families vary either side of that, so treat the diameter of an imperial size as a rough figure rather than a specification.

The decoder reads the size and the service description. It does not read the rest of the sidewall, and a commercial sidewall carries a lot more: the DOT code that gives the week and year of manufacture, the ECE type approval mark, M+S and 3PMSF winter markings, the regroovable marking, tubeless or tube type, the maximum pressure marking, TRA codes and star ratings on earthmover tyres, and the IF or VF prefix on a high-flexion agricultural tyre.

That last one is worth spelling out. If you enter IF 710/70R42 the size will parse, but the prefix is ignored — the extra load capability that makes an IF tyre an IF tyre is not applied to the figures. Use the manufacturer’s load and pressure table for IF and VF work.

Two more limits. The decoder does not check that a size exists, so it will decode a plausible-looking combination no manufacturer has ever made. And it says nothing about fitment: whether the size suits a given rim width, whether it clears the arch, or whether the vehicle is approved for it.

The full ISO speed symbol table, from the A-series used on slow-moving industrial and agricultural equipment through to the road-going letters. The decoder reads from exactly this table.