Under-inflated or over-inflated: what each one does to your car
It's the air in a tyre that carries the weight of the car, not the rubber. The pressure decides the shape of the patch where the tyre meets the road, and the manufacturer's figure is the one that keeps that patch flat and even. Go either side of it and the car behaves differently.
Under-inflated: too soft
With too little air, the tyre sags. The contact patch spreads out, the two outside edges carry most of the weight, and the middle of the tread lifts slightly away from the road.
- It wears on both edges. The shoulders of the tyre scrub away while the centre still looks fine.
- It runs hot. The sidewalls flex more on every turn of the wheel, and that heat is what makes a tyre fail at speed, especially with a heavy load on a hot day.
- It handles worse. Steering feels slow and vague, and the car leans more in corners.
- It's worse in the wet. The tread grooves partly close up, so the tyre can't clear water as well.
- It costs fuel. A soft tyre takes more effort to roll: about 0.2% worse economy for every 1 psi the tyres are down on average, according to the US Department of Energy.
- It's easier to damage. Hit a pothole and a soft tyre can be pinched between the road and the wheel rim.
Over-inflated: too hard
With too much air, the tyre bulges in the middle. The contact patch shrinks and the centre of the tread does most of the work.
- It wears down the middle. The centre of the tread goes first, while the edges look nearly new.
- It grips less. A smaller patch means less rubber on the road, which you notice most when braking hard or in the wet.
- The ride is harsher. You feel every bump, and a very hard tyre is more likely to be cut or bruised by a sharp edge.
Reading the wear
| What you see | Likely cause |
|---|---|
| Both edges worn, centre fine | Running under-inflated |
| Centre worn, edges fine | Running over-inflated |
| One edge worn (inside or outside) | Usually wheel alignment, not pressure. Worth having it checked. |
| Patchy or scalloped wear | Often worn suspension or an out-of-balance wheel |
"A bit over is safer, isn't it?"
A pound or two above the figure does less harm than the same amount under, which is why some people round up. But "a bit extra to be safe" quickly turns into a tyre that grips less and wears unevenly. The better answer is to use the right figure for how the car is being used: most cars have a normal figure and a higher fully loaded one for passengers, luggage or towing.
Warm tyres read high
Driving heats the air inside a tyre and the pressure goes up, often by a few psi. That's normal and allowed for in the manufacturer's figures, which are for cold tyres. So if you check at a petrol station after a drive, don't let air out to bring a warm tyre down to the cold figure: you'll end up under-inflated once it cools. Check again cold if you can.
The number on the tyre
The pressure moulded into the sidewall ("max press" or similar) is the most that tyre can safely hold. It isn't the figure to inflate to. Your car's own figures are on the label in the door frame or fuel flap, in the handbook, or here by registration.