How to Establish the Best Tyre Pressures for Your Car


The car and tyre manufacturers work together to establish the best tyre pressures for your car. The handbook may give several sets of pressures for different sizes of tyre fitted to various models in the range. This is because the pressures required to carry the car’s weight and cope with its performance vary according to the flexibility of the tyre.

In addition, while some cars only have one set of pressures, others’ handbooks give different pressures for normal and full loads and high speeds. Most handbooks define what they mean by these things. For example, a normal load might be up to four people with no luggage while a full load is five people and luggage. High speed generally refers to driving at speeds in excess of 80 mph for long periods.

Best Tyre Pressures How to Establish the Best Tyre Pressures for Your Car

Pressures and efficiency

Checking pressures is important because this complex component of the car can only work properly if they are correct. If tyres are under-inflated they flex too much, which at speed creates heat with the danger of the tyre breaking up. It may allow an upward curve in the tyre’s tread so the centre of the tread has less contact with the ground than it should. Together, heat and uneven tread pressure increase wear while the increased flexing of the tyre also increases its rolling resistance so it needs more of a push to rotate, which increases fuel consumption. So, apart from the safety aspect, under-inflated tyres increase running costs.

But don’t over-inflate the tyre to try to cut fuel bills because that too can increase tyre-wear by forcing an outward curve on the tread resulting in more wear on the centre. It also reduces grip by not allowing the tyre to flex enough to cope with bumps and cornering and by creating that curve on the tread. It can also seriously affect handling in other ways because the suspension is set up to work with the tyres at certain pressures. By over-inflating them you change the way it responds. Your steering might then feel over light and the car’s cornering attitude changes.

Equality

Remember, too, that four-wheel drive systems that distribute drive according to grip, and most electronic driving aids, work by comparing wheel rotation speeds. Over- or under-inflation increases or reduces the tyre’s circumference so it rotates at a different speed to a correctly inflated tyre. Incorrect pressures can affect the efficiency of these systems, especially if it is only on one wheel.

Uneven tyre pressures also often make themselves felt by the car behaving differently when it corners to the left and right. The difference may be slight, but in cars with good road feel an attentive driver will notice it and should check the pressures.

Best Tyre Pressures 1 How to Establish the Best Tyre Pressures for Your Car

Core issues

If you find a tyre on which the pressure has dropped, check the valve is not stuck or faulty. If you wipe saliva across the open end of the valve it will form a bubble if the valve is leaking. Inside the valve is a valve core, which is a tiny metal and plastic device that is held shut by air pressure and opened when a central pin is pushed down by an airline nozzle or other object. The sides of the core are threaded so it screws into the valve tube, which is also threaded. Sometimes dirt gets in and jams it partially open or the core just works loose and needs tightening. You can buy packets of them in car accessory shops and need a special tool to tighten or replace them, though some tyre pressure gauges include one. It’s a metal or plastic rod with a slot across the end and a hole in the middle: the slot grips the core’s shoulder and the hole is for the valve’s pin.

Take care when undoing the valve because the sudden release of pressure can fire debris, including the valve core and tool, into your face. When you have replaced the valve core you must completely reinflate the tyre so you need access to either a garage airline or a powered pump.

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