Car basics · First steps · about 10 minutes

Check Tire Pressure and Tread

Tires are the only part of the car that touches the road. Keeping them at the right pressure and replacing them before the tread wears out helps the car grip, stop, and save fuel. All you need is a gauge and a penny.

When to teach it: Watch from about 7, help from about 9, do it alone from about 12.

If you can see the top of his head, the tread is worn to about 2/32 inch or less and it's time for new tires.

You'll need

A tire pressure gauge (keep one in the car)
A penny
Your car's tire pressure label or owner's manual
An air pump (many gas stations have one)

Before you start

Check tires cold: the car should not have been driven for at least three hours.

Step by step

Tick each step as you go if you like; ticks stay on this device.

Find your car's numberLook for the tire label on the edge of the driver's door or on the door post, or check the owner's manual. Use that number, not the one on the tire's sidewall. The sidewall number is the tire's maximum, not what your car needs.
Make sure the tires are coldCheck before you drive, or after the car has sat at least three hours. Warm tires read higher than cold ones.
Measure each tireUnscrew the valve cap, press the gauge firmly onto the valve stem, and read the psi. Write down each tire's number.
Add or let out airIf a tire is low, add air and check again. If it's too high, press the pin in the valve stem with the edge of the gauge to let a little air out, then recheck. Screw the valve caps back on.
Don't forget the spareIf your car has a spare tire, check it too.
Do the penny testPut a penny into a tread groove with Lincoln's head upside down and facing you. If you can see the top of his head, the tread is worn out and it's time for new tires.
Look for wear bars and damageRaised bars run across the bottom of the tread grooves. If the tread is worn down level with them, replace the tires. Also look for cuts, cracks, or bulges.
Repeat every monthCheck all your tires at least once a month and before long trips.

Physics

How it works: Why cold mornings lower tire pressure

The air in a tire is made of tiny molecules zooming around. Every time they hit the inside of the tire, they push on it. All those pushes add up to the pressure your gauge reads.

When air warms up, the molecules move faster and hit the walls more, so pressure goes up. When air cools, they slow down and pressure drops. This is Gay-Lussac's law: if the space stays the same size, pressure rises and falls with temperature.

For car tires, AAA says pressure changes about 1 psi for every 10 °F change in temperature. So a 30-degree cold snap overnight can leave a tire about 3 psi low. That's why you check tires cold, before driving warms them up.

Low tires waste fuel, too. FuelEconomy.gov says gas mileage drops about 0.2% for every 1 psi drop in the average pressure of all four tires.

P₁ ÷ T₁ = P₂ ÷ T₂ (P is the total gas pressure and T is temperature measured up from absolute zero)

Try it: Blow up a balloon, tie it, and measure around its middle with a string. Put it in the freezer for 30 minutes and measure again. The cold air inside pushes less, so the balloon shrinks. Let it warm up and watch it grow back.

Keep learning: Science

Teaching it to your kids

Let your child read the gauge and write the four numbers on a card.
Make the penny test a game: is Lincoln's head hidden or showing?
Have them find the door label and compare it with the sidewall so they see why the numbers differ.
Measure on a cold morning and again after a drive, just to compare. Only adjust air when the tires are cold.

Tips

The tire pressure warning light is a backup, not a replacement for monthly checks. It comes on only when a tire is well below the right pressure.
Tires can lose air suddenly after hitting a pothole or curb, so check them after a hard bump.
Front and rear tires may need different pressures. The label tells you.
Tires age even when the tread looks fine. Some carmakers and tire makers recommend replacing tires that are 6 to 10 years old. The last four digits of the DOT code on the sidewall show the week and year the tire was made.

Common mistakes

Mistake	Instead
Filling to the number on the tire's sidewall	Use the pressure on the door label or in the owner's manual. The sidewall shows the maximum.
Checking right after a drive	Warm tires read high. Check when the car has sat at least three hours.
Trusting your eyes	A low tire often looks normal. Use a gauge.
Forgetting the spare	Check it each month along with the others.

Safe habits

Replace tires when the tread is worn down to 2/32 of an inch.
Don't drive on tires with cuts, cracks, or bulges.
Keep tires at the pressure your car's maker lists.

When to call a pro

Stop using a tire that has cuts, cracks, or bulges, and have a tire shop look at it.
If the tire pressure light flashes for about a minute and then stays on, the monitoring system itself has a problem. Have a dealer or shop inspect it.

Sources

Free how-to guides for families, with nothing to earn, pass or send in. They follow U.S. rules and sources; your owner's manual, product labels and state and local rules come first.