Tyre air pressures explained

Air in your tyres is as necessary as wheels for them to go on, but why are tyre pressures so important? We explain the science behind it and how correct tyre pressures can save you money.

You might know next to nothing about tyres but there’s a good chance you know that tyre pressures are important. Every time you fill up with fuel there’s usually a tyre inflator in the corner, reminding you that it’s been several weeks since you last checked them. However much it might be a pain, regularly checking your tyre pressures is important for a number of reasons.

Why do I need to check my tyre pressures?

Assuming your car came with tyres when you bought it there will also be information secreted somewhere on it to tell you what the tyre pressures should be. Frequently found on the driver’s door pillar or inside the fuel flap and always shown in the handbook, there are recommended pressures to suit all the different size wheels and tyres your car might have been fitted with, as well as different pressures depending on the vehicle load.

The reason pressures are so important is because they can dramatically affect how a tyre behaves on the road. When inflated correctly, a tyre will have an even contact patch so that as much of the tyre’s width is in contact with the road as possible. An over-inflated tyre will bulge slightly, reducing the contact patch to a narrower band along the centre of the tyre. Conversely, an under-inflated tyre bulges at the sides, so that the edges of the tyre are in contact with the road and the centre of the tyre is less so. In either case, incorrect tyres mean a reduced contact patch with the road - less grip, less traction, braking and reduced safety.

How often should I check my tyre pressures?

You can’t check them too often, although once a day is unnecessarily frequent. For an average mileage driver once a week should be more than sufficient, but if you drive a few hundred miles a week or more you may want to do so more regularly. If you have a tyre pressure monitoring system fitted to your car that gives a constant readout it is worth a check every day you use your car, and back this up with a check using an actual gauge once a month.

If you can’t find the relevant information in your vehicle you can search out the correct pressure via the manufacturer’s website or one of a number of independent websites such as www.tyre-pressures.com, https://www.tyresafe.org/check-your-pressures/ and https://www.oponeo.co.uk/advice-and-tips/tyre-pressure. If your vehicle is fitted with aftermarket wheels and tyres that don’t match the original specification you should contact the tyre manufacturer for recommended pressures.

How can having the right tyre pressures save me money?

A correctly-inflated tyre will not only provide you with the maximum available grip and therefore keep you safe, it will also minimise rolling resistance. An underinflated tyre requires more effort to keep it rolling, which in turn will use more fuel, so keeping your tyre pressures set at the correct levels will help to minimise fuel consumption.

You may be tempted to overinflate your tyres in order to reduce rolling resistance still further, but as well as compromising safety this will actually lead to your tyres wearing out more quickly and requiring replacement sooner. A tyre kept to the correct pressure will give you the most grip, reduce rolling resistance and last longer than one set at an incorrect pressure - so there’s no excuse for not keeping a close eye on them. 

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Ask HJ

What is the maximum I can inflate my tyres to stop them flatting when in storage over the winter?

My Mazda MX-5 is covered for the winter and wired to my battery charger, so I cannot move it. The tyres are Bridgestone Portenza 205-45R17 84W CKC4509. What is the safest maximum I can inflate them to lessen tyre flatting?
The maximum cold tyre pressure should be printed on the sidewall. If you're storing for more than a month, we'd recommend disconnecting it from the battery charger and rolling the tyres. If you really can't do that, there are some anti-flat spot cradle gadgets you can buy - although we've got no experience of using them. If you do decide to over inflate don't forget to let the extra air out.
Answered by Keith Moody
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