A slow puncture is any leak that lets pressure out gradually rather than all at once. It is easy to ignore, because the tyre still looks fine and the car still drives normally. That is exactly what makes it expensive — a tyre run under-inflated for weeks often ends up unrepairable, when the original leak would have been a twenty-minute job.
How to tell a leak from normal pressure loss
All tyres lose a small amount of pressure naturally — roughly 1 to 2 PSI a month — plus about 1 PSI for every 5.5°C drop in temperature. That is why tyres read low on the first cold morning of autumn and it means nothing.
What is not normal:
- One tyre consistently lower than the other three.
- Losing more than about 3 PSI in a fortnight.
- Needing to top the same tyre up more than once a month.
- A TPMS warning that keeps returning after a reset.
Common causes
- A thin screw or nail. The classic one. A slim shank can seal itself well enough to leak slowly rather than deflate.
- A leaking valve. Rubber valves perish; TPMS valve stems corrode and their seals fail. This is often mistaken for a puncture.
- Corroded bead seat. Very common on older alloy wheels, especially after winter salt. The tyre no longer seals against the rim.
- A porous or cracked alloy. Usually after a pothole or kerb strike.
- Rim damage from a kerb. A small flat spot on the bead flange is enough.
Note that three of those five are wheel or valve problems, not tyre problems. That is why "it must be a nail" is often wrong, and why a proper leak trace matters.
How a fitter finds it
The tyre is inflated and either submerged in a water bath or sprayed with a soap solution, and the fitter watches for bubbles across the tread, the sidewall, the bead and the valve. Once the leak is located, the decision is straightforward: repair if it is in the repairable area and within size limits, replace the valve if that is the culprit, or clean and reseal the bead seat if the rim is the problem.
Why you should not just keep topping it up
Running 20% below the recommended pressure does three things: it increases rolling resistance so you burn more fuel, it scrubs the shoulders of the tread so the tyre wears out early, and — most importantly — it builds heat in the sidewall. That heat damage is cumulative and permanent. Plenty of tyres arrive at a fitter with a perfectly repairable nail in them and a sidewall that has already been cooked.
Checking pressures properly
- Check cold — before driving, or at least three hours after.
- Use the figure on the placard in the driver's door shut, fuel flap or handbook. Not the number on the tyre sidewall, which is the tyre's maximum, not your target.
- Use the loaded figure if you are carrying passengers and luggage.
- Do not forget the spare, if you have one.
If one tyre keeps dropping, get it looked at while it is still repairable. A mobile fitter can trace and fix it on your driveway.