Checking car tire pressure: why so important?
Why should your car and bike tire pressures be checked? Your tires 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 tire 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 tire's pressure with the right equipment; a tire pressure checker costs a few quid and this job is as simple as unscrewing the dust cover from your tire's valve before firmly pressing the gauge on it to avoid air escaping. DIY maintenance doesn't come any simpler!
Tire pressure checker: which one is best?
There are several ways to check your car's tire pressure: with the machine usually found at petrol stations; by using a foot pump with a gauge; a digital tire inflator that's powered by your car's 12V socket; or a cheap tire 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 tire pressure be?
Tire 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 tire 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 tires 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 tires, 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 tire – it is the maximum the tire 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 tire is slightly deflated.
Lastly, pressure figures are determined when a tire is cold, so make sure you take your readings at least an hour after driving.
Car tire pressure monitoring sensors: how do they work?
There are two main types of tire pressure monitoring systems (TPMS): direct and indirect.
Direct TPMS uses sensors mounted within the tire 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 tires and some vehicles even have a readout on the dashboard to enable you to monitor tire pressures precisely.
Indirect TPMS uses the ABS wheel speed sensors to detect wheels that are rotating at different speeds. An underinflated tire 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 tire with an exclamation mark in the middle.
Even if your tyres look ok, always check whenever you see the warning light.
How to check tire pressure at a garage
Most petrol station forecourts have tire 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 tire pressure machines cost money to use. Check to see what coins the machine takes. Some will allow you to check your tire 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 tires 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 tire valve (you will hear a hiss as you push it on) and the tire 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 tires (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 tire pressures be checked?
Check tire pressures once a month, but more often if you're covering hundreds of miles a week.
If you keep having to top up a tire you may have a slow puncture. But it's also possible the seal between the tire'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 tire pressure monitoring system, it makes good sense to regularly show your tires 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 tires wear out eventually. A tread depth gauge (sometimes incorporated with tyre pressure gauges) can tell you exactly how much tread is left on your rubber.
Alternatively, tires 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 tire pressure is important
An under-inflated tire 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 tire 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 tire can get a lot hotter than a correctly inflated one. This not only wears the tyre down faster, but could cause the tire to fail with disastrous consequences.
And it's not just under-inflation – over inflation is also bad. An overinflated tire will have less of the tire surface in contact with the road, so handling and braking will suffer, as will tire wear, with the tire wearing out in the middle much faster than it would if it was correctly inflated. Also, overinflated tires 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 tire pressures is important.