Checking car tyre pressure: why so important?
Why should your car and bike tyre pressures be checked? Your tyres are the only point of contact your vehicle has with the road, so it's vital that you regularly keep tabs on their condition – and that includes pressures.
Having the correct tyre pressure ensures the tread doesn't suffer from uneven wear, and it can even improve the fuel efficiency of your vehicle.
Anyone can check a tyre's pressure with the right equipment; a tyre pressure checker costs a few quid and this job is as simple as unscrewing the dust cover from your tyre's valve before firmly pressing the gauge on it to avoid air escaping. DIY maintenance doesn't come any simpler!
Tyre pressure checker: which one is best?
There are several ways to check your car's tyre pressure: with the machine usually found at petrol stations; by using a foot pump with a gauge; a digital tyre inflator that's powered by your car's 12V socket; or a cheap tyre pressure gauge. We'd recommend using your own gauge rather than one at a petrol station, because public gauges can be inaccurate.
The principle is always the same. You first need to remove the valve cap (the little screw-on cap) that you'll find near the rim of the wheel – it's a Schrader valve, which is the same type found on most pushbikes.
Push the gauge onto the valve and it will give you a reading. It's normal for a tiny bit of air to escape as you push the valve on. If air keeps coming out, you've not pressed the gauge on firmly enough.
What should my tyre pressure be?
Tyre pressures vary from vehicle to vehicle, and depending on what you use the car; a fully loaded car going on a long motorway trip should have a higher pressure than a car with one person who spends most of their time driving about in town.
Pressure is usually measured on two scales: PSI (pounds per square inch), or bar (the metric equivalent). Look to your vehicle's handbook for the tyre pressure chart, or the stats may be printed on a sticker located somewhere around the driver's door frame or inside the fuel filler flap.
Generally, car tyres need to be inflated to around 30PSI, but your car is likely to differ slightly – and there'll often be a different optimal pressure for your front versus rear tyres, and the latter may need extra inflation if you often travel with rear-seat passengers or carry heavy luggage. Do not use the figure printed on the side of the tyre – it is the maximum the tyre can withstand safely and is likely to be considerably higher than the pressure you actually need.
Try to check the pressures on a monthly basis at the very least (make a note on the first weekend of each month to remind yourself), but preferably more often if it looks like the tyre is slightly deflated.
Lastly, pressure figures are determined when a tyre is cold, so make sure you take your readings at least an hour after driving.
Car tyre pressure monitoring sensors: how do they work?
There are two main types of tyre pressure monitoring systems (TPMS): direct and indirect.
Direct TPMS uses sensors mounted within the tyre that monitor the pressure levels. When the pressure drops to a pre-set level the sensor alerts the car's ECU (commonly via RF transmission) and this flags up a warning on the dashboard.
The benefit of direct TPMS is that it gives accurate readings for all tyres and some vehicles even have a readout on the dashboard to enable you to monitor tyre pressures precisely.
Indirect TPMS uses the ABS wheel speed sensors to detect wheels that are rotating at different speeds. An underinflated tyre will have a slightly smaller rolling radius and it's that which the car detects. It's a much simpler system, but less accurate, more prone to false alarms and cannot detect if all four tyres are simultaneously low (which is unlikely, but possible).
Both systems will illuminate a dashboard warning light that looks like a cross-section of a tyre with an exclamation mark in the middle.
Even if your tyres look ok, always check whenever you see the warning light.
How to check tyre pressure at a garage
Most petrol station forecourts have tyre pressure machines – they're usually next to the water filling point and a vacuum cleaner. They can be less accurate than shop-bought gauges, so we'd only use them if you have no other option:
- Most garage tyre pressure machines cost money to use. Check to see what coins the machine takes. Some will allow you to check your tyre pressures for free. If that's the case, check all five tyres (including the spare wheel) before you insert money to activate the compressor.
- Find out what pressure you need to inflate the tyres to; this will be in your car's handbook or on the inside of the fuel filler flap or inside the door shut.
- Most machines are digital and you need to select the PSI on the machine itself. So if you select 32PSI the machine will stop inflating at the correct level and usually beep at you (in case you can’t see the readout).
- Simply push the inflator onto the tyre valve (you will hear a hiss as you push it on) and the tyre will inflate automatically.
- When the correct pressure has been achieved pull the inflator off the valve (it's normal for a bit of air to escape as you remove it. Do this for all five tyres (if you have a spare in the boot) and you're good to go.
- Some older, non-digital inflators have a gauge on the actual inflator. These attach in the same way, by pushing them onto the valve. To get a reading give the lever on the inflator a quick press, then let go. The gauge should give you the correct reading. Now press the lever to inflate to the correct level, releasing it frequently to check the PSI. If you've put too much air in, release the inflator slightly, and allow a little air to escape – you'll hear the hissing noise.
- Don’t forget to refit the valve caps.
When should car and bike tyre pressures be checked?
Check tyre pressures once a month, but more often if you're covering hundreds of miles a week.
If you keep having to top up a tyre you may have a slow puncture. But it's also possible the seal between the tyre's bead and your wheel rim may not be as tight as it should be, and valves have also been known to leak. You can check for a leaking valve by smearing a little water over the end of the valve – any escaping air will form a bubble.
Any of these scenarios can be fixed by a competent tyre shop and you may not need to fork out for new rubber.
Even if your pressures remain consistent, or your car has its own tyre pressure monitoring system, it makes good sense to regularly show your tyres some love; after all, they take a lot of abuse. Nasties such as nails or screws can burrow themselves into your rubber, and occasionally a tumorous protrusion may form on either sidewall of a tyre, signifying an internal failure of the carcass. The latter is very dangerous and you should replace the tyre immediately.
Finally, even the most mollycoddled tyres wear out eventually. UK law dictates at least 1.6mm of tread depth. A tread depth gauge (sometimes incorporated with tyre pressure gauges) can tell you exactly how much tread is left on your rubber.
Alternatively, tyres have small lumps moulded into the bottom of their main radial tread grooves. When the surrounding rubber wears down level with these bumps, it's time for new boots.
Why tyre pressure is important
Under-inflated tyres
An under-inflated tyre can increase braking distances and give undesirable handling characteristics in the event that you need to swerve in an emergency.
The extra drag caused by a tyre running with low pressure will also increase your fuel consumption by a noticeable amount, plus it'll affect the car's performance and therefore the emissions.
As if that wasn't enough, a partially deflated tyre can get a lot hotter than a correctly inflated one. This not only wears the tyre down faster, but could cause the tyre to fail with disastrous consequences.
Over-inflated tyres
And it's not just under-inflation – over inflation is also bad. An overinflated tyre will have less of the tyre surface in contact with the road, so handling and braking will suffer, as will tyre wear, with the tyre wearing out in the middle much faster than it would if it was correctly inflated. Also, overinflated tyres will make the ride much harsher – it's amazing the difference just 5 PSI too much pressure can make.
Armed with that information it's pretty obvious why maintaining the correct tyre pressures is important.