At the end of the code on your sidewall there is a number and then a letter. The number is the load index and the letter is the speed rating, and the letter is the one that gets ignored. It is not a guess at how fast you are likely to drive. It is a certified statement of the speed at which that casing can carry its rated load and still shed the heat it is generating, tested to destruction, and it forms part of what the car was approved with when it was built.
Above that speed the tread stops recovering its shape quickly enough after each rotation, and a ripple forms and travels around the tyre behind the contact patch. That is a standing wave, and once it appears the tyre is being pulled apart from the inside within a mile or two. It explains why a burst on a fast road so often happens to a tyre that looked completely healthy, and why fitting a lower rated tyre than the placard calls for is a poor economy even on a car that never sees a fast road, because the rating also tracks how the casing copes with heat and load together on a warm day with everybody on board.
Once a tyre has gone, it has gone. The van carries stock for cars, SUVs and 4x4s, one call sets it moving, whether you are stopped near Sheldon Square or miles from it, and the live ETA tells you when help actually arrives. Blocked drive in Little Venice? Open kerb in Sheldon Square? Either works. Which street in Sussex Gardens, which way the car sits in Praed Street, and we are set.
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The Edgware Road (A5) runs heavy at almost every hour, and cars setting off on a long journey usually join it with everybody and everything on board. That is the load case the speed rating exists for and it is the one drivers are least aware of, because nothing about a heavy car at town speeds feels any different. Before a long run it is worth a minute with a torch reading the letters on all four sidewalls, particularly if any of the tyres arrived on the car second hand or as a roadside replacement somewhere far from home.
A car we were called out to on Westbourne Terrace had three tyres to the placard specification and one a rating lower, fitted somewhere along the way. Nothing had gone wrong yet. It was simply the corner we would have expected to fail first on a long run.
Heat is cumulative, so what finds a marginal tyre is a loaded car, warm weather and an hour of sustained speed rather than a brief burst of it. That combination is exactly what a car sets off from here to do.
What the car requires is on the placard in the door shut. What is fitted is on the sidewall. Comparing the two takes a minute and is worth doing once, because the tyre on the car is only evidence of what somebody chose last time.
It is the speed rating, and it certifies the speed at which that tyre can carry its rated load without overheating. The number immediately before it is the load index. Both figures come from the car maker's specification and both are printed on the placard in the driver's door shut.
It still does, because the rating is really a measure of how the casing handles heat and load together. A lower rated tyre on a heavy car, loaded up in warm weather, is working much closer to its ceiling at perfectly ordinary speeds than the correct tyre would ever be.
The tread stops springing back quickly enough between rotations, and a ripple begins running around the tyre behind the contact patch. Once that starts, the tyre is being destroyed from within very quickly, and the failure which follows is sudden and total rather than gradual.
Yes, that is allowed and in some sizes it is unavoidable. Going the other way is not. Keep the two on any one axle the same as each other, because a mismatched pair leaves you with two casings behaving differently at the same end of the car.
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