Air. You won't go far without it, your car's engine needs plenty of it, and your teeth would have long since rattled from their mountings had Mr Dunlop not thought to fill his tyres with a comfy cushion of the stuff.
In ye olden days tyres could leak more than Julian Assange, but tyre tech has come a long way and many drivers now take their rubber for granted.
It's still well worth keeping tabs on your tyre pressures though. A pressure gauge costs peanuts, and they're as easy to use as unscrewing the dust cover from your tyre's valve before firmly pressing the gauge on it to avoid air escaping. Pressure is usually measured on two scales: PSI (pounds per square inch), or bar (the metric equivalent).
How to check your car's tyre pressure
You can check the pressure either using the machine at petrol stations, by using a foot pump with a gauge, or a digital tyre inflator, or by buying a cheap tyre pressure gauge. We'll explain these in a bit.
The principle is always the same. You first need to remove the valve cap (little black screw on cap) that you'll find near the rim of the wheel - it's a Schrader valve, like you'd find 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 be the correct tyre pressure?
Tyre pressures vary from car to car, and depending on what you use the car for - a fully-loaded car going on a long motorway trip is advised to have a higher pressure than a car with one person who spends most of their time driving about in traffic.
Look to your vehicle's handbook for the exact PSI, or the stats may be printed on a sticker located somewhere around the driver's door frame. Generally, car tyres need to be inflated to around 30PSI, but you do need to check to make sure yours are at, or near to the correct level.
It's worth checking the pressure levels on a monthly basis at very least - preferably more often if it looks like the tyre is slightly deflated.
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.
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.
Lastly, pressure figures are determined when a tyre is cold, so make sure you take your readings at least an hour after driving.
A major no-no here is to inflate to the PSI rating moulded on your tyre's sidewall. That's just the maximum pressure the tyre can sustain before its liable to pop. Pumping up to this figure will leave your tyre bulging, increasing wear around its central section while also reducing grip and ride quality.
On the flip side, under-inflation will allow the tyre to flex too much, accelerating wear around the shoulders. The extra friction generated by a soft tyre won't do your fuel economy any favours, either.
What are tyre pressure mounting systems?
There are two main types of tyre pressure monitoring systems (TPMS) - direct, and indirect.
Direct TPMS employ 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 wheels and some vehicles even have a readout on the dash to enable you to monitor tyre pressures precisely.
Indirect TPMS use the ABS wheel speed sensors to detect wheels that are revolving at different speeds. An underinflated tyre will have a slightly smaller rolling radius and it's that that 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 use tyre pressure machines at garages
Most petrol station forecourts have tyre pressure machines - they're usually accompanied by a water filling point, and a vacuum. But how do they work?
- Check to see what coins the machine takes
- Once you've got over the fact that they charge you for air, go around your car and unscrew the little black valve caps on each of the wheels. Pop these in your pocket, or somewhere safe.
- You'll need to know what to inflate your tyres to. Normally this is around 30-32PSI. It will say the correct level in your handbook, or commonly 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 little 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 four tyres 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
- Don’t forget to refit the valve caps! If you're missing one have a look on the floor around the machine, people always leave them behind!
How often should you check tyre pressure?
We recommend you check your tyre pressures every week, as let's face it, it's hardly the trickiest task in car care.
If you do notice a gradual pressure decrease, a slow puncture could be the culprit. 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 spontaneously break a wheel's wind.
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 like nails or screws can burrow themselves a home in your rubber, and occasionally a tumorous protrusion may form on either sidewall of a tyre, signifying an internal failure of the carcass.
Finally, even the best most mollycoddled tyres wear out eventually. European law dictates at least 1.6mm of tread depth and most US states demand a similar 2/32". A tread depth gauge 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 having the correct tyre pressure is important?
The best way to explain why the correct tyre pressure is important is to describe what can happen if you run with tyres low on air. An under-inflated tyre is less safe.
It 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 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!
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.
Armed with that information it's pretty obvious why maintaining the correct tyre pressures is important!