An under-inflated tyre is one running below the recommended pressure for the car. It is the more common pressure fault, and the more dangerous of the two, because a soft tyre runs hot, and heat is what destroys tyres.
The signs
Under-inflation is not always obvious by eye, but the clues are there:
- A pressure warning light on the dashboard
- Heavy or vague steering, and a car that feels less precise
- A tyre that looks slightly flat or bulging at the bottom
- Over time, wear on both outer edges of the tread, where the under-supported tyre presses hardest
A gauge check on cold tyres is the only way to be sure, which is why a regular pressure check matters.
Why it is dangerous
The core problem is heat. A soft tyre flexes more than it should as it rolls, and that flexing builds heat in the structure. Left to run, that heat can make the tread separate from the body of the tyre and, at speed, cause a blowout, a sudden loss of pressure that can take a car out of control.
The risks go further:
- Longer braking, as the contact patch is distorted and grip falls
- A higher risk of aquaplaning in the wet
- Sidewall damage from over-flexing, which can ruin a tyre permanently
The costs
Even short of danger, under-inflation is expensive. A soft tyre has more rolling resistance, so the car uses more fuel, or loses range, in an electric car, and the edge wear shortens the tyre's life, bringing forward the day it needs replacing, and the cost of a new one, bought online from a tyre retailer like Tyres.co.uk. The economical pressure and the safe pressure are the same: the one on the placard.
What to do
The fix is simple: bring the tyres back to the recommended pressure and check what caused the drop. A tyre that keeps going down despite topping up may have a slow puncture or a faulty valve, which needs looking at rather than repeatedly re-inflating.
From the workshop: the edge-wear pattern is a giveaway. When both shoulders are worn and the middle still looks healthy, the tyre has been run soft for a long time, and that wear does not come back.
Sources and accuracy. The effects described here reflect manufacturer and motoring-body guidance at the time of writing. Anything safety-critical should be confirmed against current official guidance. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
How can I tell if my tyres are under-inflated?+
The pressure warning light may show, the steering can feel heavy or vague, and the tyres may look slightly flat at the bottom. Over time, wear concentrated on both outer edges of the tread is a clear sign. A gauge check on cold tyres confirms it.
What are the dangers of under-inflated tyres?+
Under-inflation makes a tyre run hot, which in the worst case can cause a blowout. It also lengthens braking, raises the risk of aquaplaning, makes steering vague, wears the tyre's edges, and increases fuel use. None of it is good for safety or cost.
Do under-inflated tyres use more fuel?+
Yes. A soft tyre has more rolling resistance, so the engine works harder to move the car and burns more fuel. Even being a little under the recommended pressure measurably raises fuel use over time, and the same applies to range in an electric car.
Is it illegal to drive on under-inflated tyres?+
Low pressure is not illegal in itself, but its effects can be. If under-inflation has worn the tread below the legal limit on the edges, or a warning light is on at an MOT, that can mean a failure, and the safety risks apply long before then.