Tyre pressure affects how your car brakes, corners, wears and how much fuel it drinks. It is the single most valuable free thing you can do for your car, and most drivers still get it wrong.
Why it matters
Under-inflated tyres flex more, build heat, wear out on the edges and use more fuel. Over-inflated tyres reduce grip and wear out in the middle. Both make the car less safe in an emergency stop.
Find your numbers
Do not use the number printed on the tyre itself — that is the tyre's maximum, not your car's recommendation. Look for the tyre placard, usually a sticker inside the driver's door jamb, sometimes inside the fuel flap or glovebox. It lists the correct pressure for normal driving and often a second, higher figure for when the car is fully loaded.
Check them cold
"Cold" means the car has been sitting for a few hours or driven less than a couple of kilometres. Driving heats the air inside and raises the reading, so checking after a long trip will make the tyres look fine when they are actually low. If you can only check at a servo after driving, add a few extra kPa to allow for it — or better, check on the way out in the morning.
How to do it
Unscrew the valve cap, press the gauge on firmly and straight until the hissing stops, and read the number. Add or release air until it matches the placard. Put the cap back on — it keeps dirt and water out of the valve.
Australia uses kPa on most placards, but many servo gauges show psi. Roughly 200 kPa equals about 29 psi, and 240 kPa is about 35 psi. Most gauges let you switch units.
Do not forget the spare
A flat spare is useless on the side of the road. Check it every few months. If your car has a space-saver, it usually runs a much higher pressure — check its own label.
How often
Once a month, plus before any long drive or when carrying a full load of people and luggage. Tyres lose pressure naturally over time, and cold weather drops it further.