Run-flat tyres: how they work and what to do when one fails

The short answer: a run-flat has reinforced sidewalls that support the car with no air in it, typically for about 50 miles at up to 50mph. Once run flat, it must be replaced — repairing one is almost always refused, and for good reason.

How they work

A conventional tyre relies entirely on air pressure to hold its shape. Lose the air and the sidewall collapses under the weight of the car.

A run-flat has a much stiffer, thicker sidewall that can carry the vehicle on its own. You get mobility after a puncture instead of a roadside wheel change, which is why manufacturers pair them with cars that carry no spare.

The limits are real

The typical figure is 50 miles at 50mph, but check your handbook — it varies by manufacturer and tyre. Those numbers are not conservative padding. Running flat generates enormous heat in the sidewall, and beyond the rated distance the structure breaks down.

You also need TPMS to work. Without a pressure warning you may not notice a run-flat has deflated: they do not go visibly soft and the car barely changes. Drivers have covered hundreds of miles on a flat run-flat without realising.

Why they are not repaired

• Once driven on flat, the internal structure is compromised in ways that cannot be inspected reliably.
• There is no way to know how far or how fast it was driven before it reached you.
• Most manufacturers explicitly prohibit repair.

If a run-flat has picked up a nail and has not been run flat, some can be repaired within the usual rules. It depends on the manufacturer and the damage, and it is a conversation to have with the fitter rather than an assumption.

Can you switch to normal tyres?

Often yes, and many owners do — run-flats ride firmly and cost more. But be clear about what changes:

• You will have no mobility after a puncture, and the car probably has no spare. Carry a sealant kit or accept you are calling someone.
• Change all four. Mixing run-flats and conventional tyres gives noticeably different behaviour side to side.
• The ride usually improves.

Going the other way, to run-flats on a car not designed for them, is not advisable: the suspension was not tuned for the stiffer sidewall, and you need working TPMS.

If you are on a run-flat now

Slow down, keep within the rated distance, and get it sorted rather than stretching it. We carry run-flats in common sizes — call 07989 606060 with the size and the markings off the sidewall.