Speed symbols: an alphabet that is not in order

The final character of a service description is the speed symbol. It records the highest speed at which the tire sustained a standardised load in a standardised test, and it is defined in kilometres per hour. Every mile-per-hour figure in the table below this text is a conversion of the metric value, computed rather than typed.

What the test measures

The tire is mounted on a test rim, inflated, loaded against a large steel drum and run at increasing speed in steps — conventionally 10 km/h at a time, held for ten minutes each — until it reaches the speed being claimed and completes the final step without failing. It is an endurance test at a fixed load and a controlled temperature, indoors.

Three things follow from that, and they are the useful part:

• The symbol is tied to the tire's load index. Above the rated speed, the load figure is simply not applicable — the two are certified together, not separately.
• The test assumes correct inflation and an undamaged tire. Industry repair guidance treats a punctured-and-repaired tire as no longer carrying its original speed certification.
• It is not a recommendation, a permission, or a statement about any road anywhere.

Why H is out of position

H was standardised long before the rest of the letters, back when it sat at the top of a much shorter scale. When the series was extended, the new symbols were assigned in alphabetical order through the lower speeds — L, M, N, P, Q, R, S, T, U — and by the time the sequence caught up with H, moving it would have invalidated every tire already marked with it.

So H stayed where its speed put it, at 210 km/h, wedged between U at 200 and V at 240. It is the only letter in the series whose position has to be memorised rather than deduced.

The Z problem

Z was originally not a step but a ceiling: "above 240 km/h", with no upper bound stated. As road cars kept getting faster, that became useless as a specification, and W and Y were added with definite values.

The legacy is still visible in two places. A size may be written 245/40ZR18, with the Z sitting in the construction position rather than in the service description — a hangover from when Z was declared as part of the size. And a service description may read (Y) in parentheses, which denotes a tire tested above the plain Y value. Where a modern tire carries both, the service description is the specification and the ZR is decoration.

Where each symbol is actually encountered

The table below states this per row, but the pattern is worth naming: the low end of the series belongs to temporary spares, off-road and heavy light-truck tires, where load capacity and casing strength are bought at the cost of speed. The middle — S, T and H — covers the overwhelming majority of family cars, vans and crossovers. V and above belong to performance tires, where the casing and compound choices that support the rating tend to shorten tread life.

An all-terrain light-truck tire commonly carries R or S; the same vehicle's road-biased highway tire may carry T or H in the same size. The difference is real and it is in the construction, not in the marketing.

Winter tires and the symbol

Winter tires frequently carry a lower speed symbol than the summer or all-season tire they replace, because the softer compound and deeper tread generate more heat. Some European jurisdictions require a reminder of the lower figure to be displayed in the driver's view when a tire rated below the vehicle's maximum speed is fitted. Rules differ by country and change; the marking itself does not.

Reading it with the load index

A service description binds the two: in 104T, 104 is the load index and T the speed symbol, and neither is valid without the conditions of the other. The load index table covers the first half, and the load and speed index decoder will take the whole string and return both.

The rows below are generated from the standard series, so the site's decoder tool and this page cannot disagree. Read the mile-per-hour column as a conversion rather than a specification: the certified value is the metric one, and a symbol certified at 210 km/h converts to a shade over 130 mph rather than to a round number anybody chose.