Tire tread, pressure, and cold air dropping your numbers
Okay. Last time we did the whole "how do you know a wiper's actually shot" conversation. Same idea today, different part of the car. Tires.
Tires are the one thing between you and the road, and up here in the winter that's not a throwaway line. Point of the mountain gets its own weather half the time, and if your tread's marginal, that's exactly where you find out.
The penny test, and why I actually don't love it
Everybody knows the penny trick. Lincoln's head, upside down, into the tread grooves. If you can see all of his head, tread's too low. It works, it's fine, I'm not going to tell you not to do it.
But here's the thing — it only tells you "illegal," not "good for winter." Legal tread depth is about 2/32 of an inch. That's the bare minimum to not get pulled over. For snow and slush, you want more like 5/32 or 6/32 if you can get it. So honestly, flip the penny around and use the back, where Lincoln's whole head disappears sooner, or just buy a $6 tread depth gauge and stop guessing. Same instinct as me telling you to weigh flour instead of scooping a cup — a number is a number, an impression is a guess.
How to actually check it:
- Check all four tires, not just the ones you can see easy in the driveway. Rear tires wear different than front ones, especially if you've got a front-wheel-drive car.
- Check in at least two spots on each tire — inside edge, middle, outside edge. Uneven wear across those spots usually means an alignment or pressure problem, not a tread problem.
- Look for the wear bars too — those little raised rubber nubs built into the grooves. When the tread's flush with those, you're done, no gauge needed.
Pressure — and why winter makes your light lie to you a little
Every car's got a recommended tire pressure, usually a sticker on the inside of the driver's door jamb. Not what's printed on the tire sidewall — that's the max, not the target. Door jamb number is the one you want.
Here's the part that trips people up every single year around here. Air contracts when it's cold. For roughly every 10 degrees the temperature drops, you lose about 1 PSI. We can swing 40, 50 degrees between a warm October afternoon and a January morning, easy. That's your tire pressure light coming on not because something broke, but because the air itself shrank. Super common, and super fixable, you just have to actually check it instead of ignoring the light for three weeks and hoping it goes away.
That's most breakdowns and most flat-out-front-tire embarrassments, honestly — not bad luck, just somebody ignoring a small signal because it wasn't screaming at them yet. The car was talking. Nobody was listening.
Checking and filling pressure, start to finish:
- Check pressure cold, before you've driven anywhere, or at least three hours after driving. Driving heats the air in the tire and gives you a falsely high reading.
- Unscrew the little cap on the valve stem, don't lose it in the gravel, press the gauge on firm and straight until the hiss stops, read the number.
- Compare to the door jamb sticker. Low, add air. High — rare, but happens on a hot day — let a little out by pressing the little pin in the center of the valve stem.
- Gas stations still mostly have free air pumps, or Costco if you're already getting gas and tires there. Costco's automated ones are decent, honestly.
- Recheck after filling. Don't just trust the pump's own gauge, they're not always accurate, and that's exactly the "buy the decent tool" thing again — a cheap gauge that lies to you is worse than no gauge, because at least then you know you're guessing.
I taught my nephew Maverick to check his own oil last spring — same kind of task, get-your-hands-a-little-dirty, look-at-the-thing energy. He pulled the dipstick out just fine, then wiped it clean on his shirt instead of grabbing a rag off the bench two feet away. I just watched it happen. Didn't say a word. You only wipe a dipstick on your own shirt once in your life, and it teaches you faster than I ever could. Same thing with tire pressure — check it wrong a couple times, forget the cap on the valve stem, and you learn the routine fast.
One real caution here
If a tire's noticeably lower than the other three, or you've got a slow leak that keeps coming back every week or two, don't just keep topping it off forever. That's usually a nail or a bad valve stem, and driving on a slow leak in cold weather is how you end up with a full flat at the worst possible time — dark, cold, shoulder of the freeway. Get it looked at.
Before next time
Go check all four tires this week — tread and pressure both, cold, before you drive anywhere. If the pressure light's already on, don't be surprised if it's just the cold doing exactly what I said it does.