Be careful not to confuse the recommended pressure with the maximum pressure. The recommended pressure is the one you should use when filling your tyres and, as explained above, you can find this on your door jamb or in your owner’s manual. The maximum pressure, on the other hand, is usually stated on the sidewall of the tyre itself in small print near the tyre’s bead (where the rubber abuts the rim). This measurement is provided by the tyre manufacturer rather than the car manufacturer and is the maximum amount of pressure that the tyre can safely withstand.
What happens if you inflate your tyres to the max pressure?
Your tyres’ maximum pressure almost always exceeds the recommended pressure.
It isn’t advisable to fill your tyre to this pressure for everyday driving. At maximum pressure, your car does not handle as well, braking is impaired and you could risk dangerous blowouts. Over-inflation
can also cause the centre of your tyre’s tread to wear out prematurely and reduce the lifespan of your tyres. You may want to use the max. pressure on a temporary basis if you are hauling an especially heavy load or towing something.
Most manufacturers do not state a minimum tyre pressure, per se. The recommended value
is the amount of air that a tyre needs for a car to handle properly and safely, and anything under that value is not recommended
, so functionally the optimal value is also the minimum. Under UK law, vehicles are required to be equipped with tyre pressure monitoring systems (TPMS) that warn the driver when tyre pressure falls below 25 per cent of the recommended pressure. This is considered severe under-inflation
but less drastic pressure drops are still considered moderate or mild under-inflation and they and can still have negative consequences.
What happens if you inflate your tyres to the minimum pressure?
If your tyres are inflated to a pressure that is below the recommended PSI (as shown in the manual or on the driver’s side door jamb), your car will not operate safely. Under-inflation can cause the tyre to heat up excessively, which can make the tread pull away from the body of the tyre.
If this happens on the motorway, it could lead to a blowout and a serious accident.
There are also financial reasons for making sure that your tyres aren’t under-inflated. A low tyre has more rolling resistance,
which means that the car has to work harder to move down the road and consumes more fuel.
Well-inflated tyres are going to save you money at the pumps. Low tyres also wear more quickly and unevenly, so you will have to replace them more often.
TPMS and tyre pressure
A tyre pressure monitoring system (TPMS)
uses either a direct sensor inside your valve stem or a combination of both software and other existing sensors in your vehicle to let you know when your tyre pressure is outside a safe range
of pressures. If the pressure falls below a manufacturers specified limit (usually 25% of recommended pressure) an indicator light shaped like the letter U with an exclamation mark inside it (representing the cross-section of a tyre) will light up on the dashboard. When you see this TPMS light on
, you should immediately check your tyre pressure and make any necessary adjustments.
Evan if the vehicle is fitted with a TPMS, the vehicle manufacture handbook will advise owners to perform regular manual tyre pressure checks.