The tire pressure light is a late warning

Quick answer: Find the pressure on the sticker inside the driver's door. Once a month, check all four tires and the spare with your own gauge, cold, and fill to that number. Then put a quarter in the tread. If you can see the top of Washington's head, start shopping.

The first cold morning in October, a little yellow horseshoe with an exclamation point lights up on half the dashboards in the country. Most people drive to work anyway, mention it to somebody in the break room, get told "oh, that's just the cold," and wait for the afternoon sun to make it go away.

It usually does go away. That's the problem.

That light is the tire pressure monitoring system, required on new passenger vehicles since the 2008 model year. The federal standard only requires it to warn you once a tire is 25 percent below the recommended pressure. On a car that wants 35 psi, that's around 26. So when it finally lights up, you've been driving on a soft tire for a while. And the cold didn't cause it. Tires lose air slowly through the rubber all the time, roughly a pound a month, and pressure drops about 1 psi for every 10 degrees Fahrenheit the temperature falls. The cold front just exposed a tire that was already low.

A low tire runs hot, grips worse, stops longer on wet pavement and burns more gas. You fix all of that with a gauge and two minutes.

The number on the tire is the wrong number

Molded into the sidewall you'll find something like "MAX PRESS 51 PSI." That's the most the tire can safely hold. It's not what your car wants.

Your number is on a sticker on the driver's door jamb. On some cars it's inside the fuel door or the glove box, and it's always in the owner's manual. Most sedans and crossovers land somewhere in the low-to-mid 30s. Trucks often run higher, and some cars want different pressures front and rear. Take a photo of that sticker so you're never guessing at a pump.

Fill to the sidewall number and you get a hard, skittish ride, less rubber touching the road, and the center of the tread wearing out early. People do it every day, usually because somebody helpful at a gas station told them to.

Check them cold

Driving heats tires up, and hot air reads high. After twenty minutes on the highway, a tire can read several pounds above what it showed in your driveway. The door sticker number is a cold number.

So check in the morning before you go anywhere, or after the car has sat for three hours.

The annoying part: the air machine is usually a few miles away, and by the time you get there the tires are warm. Easy fix. Check at home first and write down how low each one is. Say the front left is 4 pounds under. At the station, add 4 pounds to whatever it reads warm. Don't let air out of a warm tire to hit the sticker number. You'll end up low by morning.

The two minutes, step by step

1. Unscrew the valve cap and put it in your pocket. Set it on the ground and it'll roll toward the nearest drain.
2. Press the gauge straight and firmly onto the valve stem. A quick hiss is normal. A long one means you're on at an angle.
3. Read it. If the number looks strange, do it again.
4. Do the other three tires.
5. Then the spare.

Everybody forgets the spare. A compact spare often wants a much higher pressure than your road tires, frequently 60 psi, and that number is printed right on it. A flat spare is just a heavy, disappointing thing in your trunk.

Find out whether you even have one. Plenty of newer cars come with a can of sealant and a small compressor instead. You want to learn that in your garage, not on the shoulder of the interstate at dusk.

At the gas station machine, the digital ones let you set a target and they beep when you hit it. The old dial ones have gauges that have been dropped a thousand times, so use your own between bursts. Some states require stations to give free air to customers who buy gas. Ask inside before you feed it quarters.

Tread: the penny and the quarter

Grab a penny. Put it into the shallowest-looking groove with Lincoln's head pointing down. If you can see the top of his head, the tread is at 2/32 of an inch or less, which is the legal minimum in most states. That tire is finished.

Now try a quarter, same way. The top of Washington's head sits at about 4/32. If you can see all of it, you're there, and a lot of tire professionals will tell you that's when to replace, especially if you drive in rain or snow. Grooves are how water gets out from under the tire. Shallow grooves can't move enough of it, and the car starts floating on the road instead of gripping it.

You'll also see small raised bars running across the grooves. Those are wear indicators. Once the tread is flush with them, you've hit 2/32.

Check each tire in three spots: the inside edge, the middle and the outside edge. Uneven numbers mean something.

What the wear is telling you

• Worn down the center, fine at the edges: overinflated. Usually the sidewall-number habit.
• Worn on both edges: driven underinflated for a long time.
• Worn on one edge, often the inside: alignment. The inside edge is hard to see, so run your hand along it.
• Scalloped dips around the tread: worn suspension parts or an out-of-balance tire. That's a shop visit.
• A bulge or bubble on the sidewall: stop driving on it and put the spare on. The internal structure is damaged, and it can fail without warning.

Rotating your tires evens out the wear. Your owner's manual gives the interval. Many say every 5,000 to 7,500 miles, which conveniently lines up with a lot of oil changes.

Age, the shop counter, and other people's cars

Tread isn't the only clock. Find the DOT code on the sidewall. The last four digits are the week and year the tire was made, so 3419 means the 34th week of 2019. Rubber hardens and cracks with age, even on a car that barely leaves the garage. Many automakers and tire makers recommend replacement somewhere between six and ten years no matter how much tread is left. Your manual will say.

Sooner or later somebody at a counter will tell you that you need four new tires. Maybe you do. But now you own a quarter. Go look. Ask them to show you the tread depth readings and which tire is the problem. A good shop will happily walk you out to the car. If they get impatient about it, that's information too. You're not being difficult. You're the customer, and it's your money.

If they offer a nitrogen fill for an extra charge, you can pass. The air you're already breathing is about 78 percent nitrogen.

To be fair to the other side: if your car came with a service plan that includes rotations and pressure checks, use it. It's free and they'll catch things. But they see the car twice a year. You see it every day.

And when you borrow a car, from your parents or a friend or your sister, check the tires before you hand it back and fill any that are low. Return it with a full tank, too, even if it came to you on fumes. Nobody will mention it, but they'll remember. Then teach the same two minutes to your kids before they drive off to college with a car you paid for.

What we'd buy, what we'd skip

Buy:

• A simple digital tire gauge for the glove box. Easy to read in bad light.
• A pencil gauge as a backup. They're cheap, they don't need batteries and they're more accurate than people expect.
• A small 12-volt inflator that plugs into the car's power outlet. You fill up cold in your own driveway, which takes care of the warm-tire problem completely.

Skip:

• Nitrogen upgrades.
• Treating the dashboard readout as your only check. Many cars show each tire's pressure, and that's handy, but put a real gauge on them now and then.
• Tire shine. It does nothing for safety.

Next October, when that little horseshoe lights up across the parking lot at work, yours won't. You'll have topped them off the Saturday before.