The correct tyre pressure is important for two reasons:
- Safety - Can be reduced due to the higher risk of over/understeer, increased braking distances and higher risk of aquaplaning
- Cost reduction - tyres with incorrect pressure will have a reduced life span and increase fuel consumption
Checking tyre pressure
Halfords Autocentre recommends checking tyre pressures (including the spare tyre) monthly 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.
Understeer: The car turns less than expected
Oversteer: The car turns more than expected
Low pressure and the effect 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.
Low pressure and the effect on braking distances
Braking distance in the wet increases by up to 11m; that's more than the width of two pedestrian crossings and could easily be the difference between hitting something and not.
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 chaning the tyres up to 5,000 tyres 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.
Halfords Autocentre recommend that you check tyre pressures weekly, or at the very least, monthly.