A nail in the tyre does not always mean buying a new one. Plenty of punctures can be repaired safely and permanently, but some can't, and it matters which is which.
Where the damage is
Only damage in the central three-quarters of the tread can be repaired. Punctures in the shoulder or sidewall flex constantly and cannot be repaired safely, so the tyre has to be replaced.
How big the hole is
The British Standard for puncture repair, BS AU 159, allows repair of damage up to 6mm in diameter. Anything larger means replacement.
What a proper repair involves
The wheel is removed, the tyre is taken off the rim and inspected inside for hidden damage, and the puncture is sealed with a combined plug and patch from the inside. An outside plug on its own is not a safe permanent repair.
When a tyre can't be repaired
Tyres that have been driven on while flat, that are worn below the legal limit, that have already been repaired in the same area, or that have sidewall damage should be replaced instead.
Slow punctures
A tyre that loses a little pressure every few days is often a small puncture, a leaking valve or a poor seal on the rim. Getting it checked early often means it is still repairable.
Common questions
How long does a puncture repair take?
Usually around 30 to 45 minutes for one tyre once the fitter is with you.
Can run-flat tyres be repaired?
Many manufacturers recommend against it, especially if the car was driven while flat. It depends on the inspection.
Is it safe to drive on a repaired tyre?
Yes, a correctly repaired tyre can be used normally for the rest of its life.
Should I carry on driving with a slow puncture?
Check pressure and get it looked at soon. Driving on an under-inflated tyre damages it and makes repair less likely.