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Checking tire pressures on the average vehicle is simple. All you need is a pressure gauge.

Most tires have a maximum rated pressure of about 40-50 PSI (Pounds per Square Inch), but most vehicles only require about 35 or so - check the sticker in the glove compartment or on the doorjamb of the driver's door to find yours.

Now that we know the proper pressure, we can proceed to checking the tire. Somewhere on the rim of the tire there will be a valve stem, usually covered with a little screw-on cap. Unscrew this cap, and set it aside. I'll assume for the moment that you have the 'stick' type of pressure gauge. Press the business end of the gauge against the vale stem such that you hear a rush of air. Continue pressing harder until the sound stops. If the sound doesn't stop, either you're using the wrong side of the gauge, or your gauge is broken. After the whooshing sound stops, check the reading on the gauge. This is your tire pressure.
Buy a tire gauge at any auto parts store, or department store or wherever.

Make sure the tires are cold--i.e., you haven't driven the car in quite a while, certainly not for long distances.

Take the valve cover off the valve of a tire.

Press the tire gauge onto the valve quickly and firmly so that little or no air escapes from the tire and the tire pressure pushes the gauge out.

Remove the gauge from the tire, and look at how far the indicator is pushed out. Match up the edge of the gauge with the closest line. The number buy that line is the tire pressure in PSI.

To find the recommended tire pressure for your vehicle, look inside the door frame of the driver's side door. There should be a sticker with both the vehicle carrying capacity and the recommended tire pressures for both the main tires and the spare tire, which often is a compact spare and needs a higher tire pressure. Some cars need different pressures in the front and back.

If you're missing the sticker, hopefully your owner's manual has the information. Please note that the recommended pressure doesn't have much to do with what kind of tire is on the car, it's mostly a function of the car's weight and balance--although the recommendations are usually for tires that are the same size as the factory-installed ones. If you change your tire size for some reason, you should know what you're doing.

An often neglected point in vehicle maintenance is the pressure of the tyres fitted to your car. Checking your tyre pressure is something that should become a habit, as it is of vital importance to the safe running of your car. After all, the only things keeping you driving in a straight line, or preventing you from going straight when you're supposed to be turning, are those thin strips of rubber strapped to your wheels. Isn't it in your best interests to ensure that they're performing to their maximum ability?

Why tyre pressure is important

Your tyre pressure affects many aspects to do with the safe handling, fuel efficiency, and ride comfort of your car - as well as how quickly your tyres will wear. Different things could happen, depending on whether they are over, or under inflated.

Under Inflated Tyres

Not having enough air in your tyres has a number of effects on your car, including:
  • A drop in handling - when you don't have enough air in your tyres, the sidewalls are allowed to flex more than they're supposed to. Instead of a reasonably rigid sidewall, they flex, and you have a more difficult time turning. Your steering will not be as precise as it could be.
  • Uneven wear - under inflated tyres wear more on the outside of the tyre, the centre of the tyre having less contact with the road. Not only will this wear the outside of the tyre more quickly than the entire tyre would wear normally - meaning you have to change them more regularly - it is also a major contributor to...
  • Reduced traction - particularly in the wet. The tread in the centre of the tyre will not be performing as well as it should be, less water is dispersed from underneath it, and the danger of aqua-planing increases. This also affects your stopping distance.
  • Poor fuel efficiency - you'll be spending more to go the same distance, yet another hip-pocket reason to make sure you're paying attention to how happy your tyres are.

Over Inflated Tyres

Having too much air in your tyres also leads to undesirable results. Some are very much the same as the dangers under inflated tyres produce:

  • Uneven wear - over inflation leads to the tyre bulging in the centre, and wearing more quickly there. The results are pretty much the same as for under inflation - yes, including...
  • Reduced traction - particularly in the wet. In this case, the outside of the tyre's not being allowed to do its job. Less tyre on the road is a bad thing when you're trying to keep as much rubber between you and the bitumen as possible. The higher pressure can also increase the risk of a skid under sudden braking.
  • A less comfortable ride - all those little bumps on the road will seem much larger when your tyres aren't absorbing as much of the impact. Instead, the force is transferred to the car's body, and through that, you.

Checking Tyre Pressure

Checking tyre pressure is easy, takes no more than a minute or two, and you need know nothing more than how to read a number. Before starting, make sure you know what your car's recommended tyre pressure is. This information will be in your car's manual, it's often also printed in the car's glovebox, or another place on the interior. When you measure your car's tyre pressure, you want the numbers to be the same as these recommended figures. Note that sometimes, there are different pressures required for the front and rear tyres. There may also be slightly higher recommended pressures for when the car is under a heavy load - keep this in mind if you've packed the car with gear and people before a road trip!

I'd seriously suggest investing in a good quality tyre pressure gauge. I'd seriously suggest not trusting the gauges on service station air hoses! Pressure gauges are sensitive, and prone to giving you a false reading when mistreated. So when someone pumps their tyres up, then drops the hose on the ground, they're helping to ensure that the reading you get from that gauge is wrong. I'd recommend the type that look like a pen, with a central core that pops out when under pressure. They're dead easy to use and read, and small enough to keep in the car glove box. If you can find a decent one that's made entirely of metal, you'll have yourself a cheap, useful tool that will last you for years. Try to not forget that you have the thing, and check your tyre pressure once or twice a week. Not only will you be safe in the knowledge that you're running on a correct pressure, but it may just alert you early on to any slow leaks your tyres have developed. Much better to discover this in the driveway at home, than on the side of the highway.

Check your tyres when they are cold - this is important if you want to get it right. When you drive, your tyres produce heat. Some of the heat is generated by the friction of the tyre on the road, but the greater amount comes from the flexing of the sidewall. The sidewall flexes when you corner, brake, go over bumps, etc. When your tyres get hot, the air inside gets hot too. So the pressure increases. You take the pressure, and end up with a false reading. In the worst case, you may actually reduce the pressure based on this false information, and reduce an already low pressure even further. I mentioned earlier that under inflated tyres allow for more side wall flexing - this leads to under inflated tyres heating up more quickly too. Just another reason to get yourself a decent pressure gauge for the car - you can measure the pressure at home, when they're nice and cold.

If you need to put more air into them, and need to visit a service station to do this, drive slowly and gently, to attempt to minimise the heat you put into the tyres.

Lastly, don't forget the spare!

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