Video - Tyre Pressure

Tyre Pressure Guide

Tyre pressure should really be checked every few weeks to ensure your vechile is still safe to drive and your tyres are not losing air.

The correct tyre pressure is important for two reasons

  1. Safety - Can be reduced due to the higher risk of over/understeer, increased braking distances and higher risk of aquaplaning
  2. Cost reduction - tyres with incorrect pressure will have a reduced life span and increase fuel consumption

Checking tyre pressure

Halfords recommend checking tyre pressures (including the spare tyre) every few weeks and before any long trip. Tyres should be checked regularly as they are susceptible to drops in pressure which can be caused by:

  • Natural leakage of air through the walls of the tyre
  • Drops in ambient temperature
  • Slow puncture

Tyre pressures should be checked cold (tyres not having run for at least two hours, or having run for less than two miles at low speed). If tyre pressures are checked hot, add 4 to 5psi (0.3 bar) to the recommended pressures. Correct the tyre pressures if they are no longer correspond to the pressures recommended by vehicle manufacturer.

Information on the recommended tyre pressures can be found in the vehicle documentation, and often on a sticker fixed to the vehicle, for example on the door jamb or on the fuel filler cap.

In case of unusual pressure loss, have the internal and external condition of the tyre checked, as well as the condition of the wheel and valve.

Low pressure affects the balance of the vehicle

  • A reduction in pressure on the front axle will tend to increase the chances of understeer.
  • A reduction in pressure on the rear axle will tend to increase the chances of oversteer.
low tyre pressure causing understeer

Understeer: The car turns less than expected

high tyre pressure causing oversteer

Oversteer: The car turns more than expected

Low pressure and the affect on aquaplaning

Michelin conducted tests to see how much of the tyre is in contact with the ground when it was progressively deflated by driving a car through a fluorescent dye on a toughened glass window and photographed the results. As you can see below, the lower the pressure, the less the tyre is in contact with the road. This will dramatically increase the chances of aquaplaning in wet weather.

If the tyre with correct pressures has a nominal surface contact area of 100% then the progressive reduction in contact can be seen.

2 bar tyre pressure

Pressure = 2 bar
Surface contact = 100%

1.5 bar tyre pressure

Pressure = 1.5 bar
Surface contact = 50%

1 bar tyre pressure

Pressure = 1 Bar
Surface contact = 25%

Low pressure and the risk of rapid deflation

Prolonged running at reduced pressure causes a build up of excess heat in the tyre, and in exceptional cases, can cause the tyre to fail and cause a rapid deflation.

Low pressure and the effect on tyre life

A tyre that is under-inflated by 20% will have a reduction in life of just over 20% (depending on use). This could mean changing the tyres up to 5,000 times sooner than normal.

Conversely, over-inflation will also increase the incidences of abnormal wear and these will be particularly accentuated in the middle of the tyre.

Low pressure and the effect on your fuel efficiency

A set of under-inflated tyres will cause the engine to work harder and this will increase your fuel consumption. A set of tyres that is 10psi under-inflated will have the same effect as increasing the cost of fuel by 4.5p per litre (based on £1.50 per litre). This could lead to needing an extra tank of fuel per year.

What is the correct tyre pressure for my car?

You can usually find out the recomended tyre pressure by checking your car handbook or by checking with your manufacturer. Other places this can be found on your vechile include

  • Your vehicle’s handbook
  • Stamped into the sill of the driver’s side door
  • Inside the fuel cap

How to check tyre pressure

To ensure you get an even reading of your tyre pressure you need to ensure your vehicle is on a flat surface and your tyres are cold as this will produce more accurate results.

Then you can either use a home tyre pressure gauge or visit a petrol station which typically have a air station and will be able to show you the pressure for each tyre.

Don’t forget to inspect your spare tyre too. This tyre can lose pressure even when it isn’t being used.

Are tyre pressures checked in an MOT?

Tyre pressures are not checked within a standard MOT test, however if you’re vehicle is fitted with TPMS (tyre pressure monitoring system) this will result in a failed MOT.

Halfords Autocentre recommend that you check tyre pressures weekly, or at the very least, monthly.