Underinflated or overinflated tyres: which is more dangerous?

Underinflated or overinflated tyres: which is more dangerous?

The short answer: both are dangerous, because tyre pressure below or above the car maker's recommended value affects handling, braking distance, road grip and tyre life.

If you have to rank them by urgency, “very low tyre pressure” is usually the bigger short-term concern, because the sidewalls flex a lot, heat builds up, and the tyre structure can be damaged to the point of losing pressure rapidly. That said, overinflated tyres are not safe either, because they reduce grip and shock absorption.

What happens when tyre pressure is too low?

When tyre pressure is below the standard value, the tyre squashes and flexes more than normal, especially at high speed or under a heavy load, which creates the following risks:

• The sidewalls work harder and heat builds up
• Steering response is slower
• The car is harder to control, especially when changing lanes or braking
• The tread on both shoulders wears faster than normal
• Rolling resistance increases, using more energy or fuel
• If the pressure is left very low and the car is driven on continuously, the tyre structure may be damaged

If one tyre loses pressure faster than the others, check for a nail, a leaking valve or wheel damage. Don't just top it up and keep driving without finding the cause.

What happens when tyre pressure is too high?

Inflating above the car maker's recommended value does not make the car safer. Although the car may seem to roll more freely or the steering may feel lighter, there are several drawbacks:

• Less of the tread is in contact with the road
• Grip and braking performance may be reduced
• The ride is harsher and absorbs less shock
• The centre of the tread may wear faster than the rest
• Tyres and wheels take more impact from potholes and kerbs

Overinflated tyres won't necessarily damage the suspension straight away, but the car will take more vibration and the ride will be less comfortable.

Summary of the differences

What PSI should your tyres be?

There is no single number that works for every car, because tyre pressure depends on the car model, tyre size, wheel position and load.

Check the pressure recommended by the car maker in these places:

1. The sticker on the driver's door pillar
2. Inside the fuel filler flap on some models
3. The owner's manual

Front and rear values may differ, and some models specify different values for normal use and for a full load of passengers or luggage.

Don't use the number on the tyre sidewall as the pressure to inflate to, because that number is usually the maximum pressure the tyre can take, not the right pressure for your car.

How to check tyre pressure accurately

• Check when the tyres are cold, before setting off or after the car has been parked for several hours
• Use a tyre pressure gauge that meets recognised standards
• Check every tyre, including the spare
• Check at least once a month and before long journeys
• If you've just been driving, don't let air out just because the reading is higher than usual, because pressure rises when tyres are hot
• Check again once the tyres are cold

Cars with TPMS still need checking with a gauge, because the warning light usually comes on only once the pressure has already dropped a lot. It is not a substitute for a monthly tyre pressure check.

What should you do if the TPMS light comes on?

Slow down, find a safe place to stop and check the pressure in every tyre. If a tyre is flat, torn, bulging or losing pressure rapidly, don't keep driving on it. Contact a service centre or an emergency assistance service.

What does incorrect tyre pressure have to do with car insurance?

If tyre damage causes an accident, take photos of the tyres, the scene and the damage to the car, and contact your insurer straight away. Whether you are covered depends on the cause, the type of policy and each insurer's conditions, so don't decide for yourself whether you can claim before the insurer has checked.

Frequently asked questions

Underinflated or overinflated tyres: which is more dangerous?

Both are dangerous, but very low pressure usually carries a more urgent risk from heat build-up and damage to the tyre structure, while overinflation reduces grip and shock absorption.

Should every saloon car be inflated to 32 PSI?

No. Each model may specify different pressures, including different front, rear and fully loaded values. Use only the value on the car's sticker or in the manual.

Can I go by the number on the tyre sidewall?

It shouldn't be used as your normal inflation pressure, because the number on the sidewall is usually the tyre's maximum pressure, not the value the car maker recommends for everyday use.

If my car has TPMS, do I still need to check the tyre pressure?

Yes, at least once a month, because TPMS may only warn you once the pressure has dropped a lot, and it cannot replace checking with a gauge.

Contact Prakanthai

To ask about car insurance cover, check your policy, request a quote or get advice after an incident, contact the Prakanthai.com team:

• Hotline: +66 2 459 2223
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