Tire pressure low when cold? Here’s why

It never fails. After the first freeze of the year, the tire light goes on in the car. Frequently both of the family cars. And if it only happens to one, it’ll happen to the other when the first really deep freeze of winter hits. Low tire pressure when cold isn’t just your imagination. Here’s why it happens and how to know if it’s a 75-cent problem or a $200 problem.

Every 10 degree drop in temperature can lower your tire pressure by about a pound, which can cause low tire pressure in cold weather. Don’t drive on a tire with less than 20 pounds of pressure in it.

Low tire pressure when it gets cold

tire pressure low when cold
If your tire pressure goes low when it’s cold, it’s not your imagination. Your tire pressure can be a few pounds low even if it the tire looks OK. If you have less than 20 pounds of pressure in any tire, don’t drive on it. Change the tire or call for help.

Cold weather makes tire pressure go low because of basic science. The science of expansion and contraction. When water freezes, it expands, and that’s one of the things that can cause cracks in your driveway or sidewalk. When air gets cold, it does the opposite. It contracts, and that lowers the pressure. Each 10 degree drop in temperature can lower your tire pressure about a pound.

When you take your car in for an oil change, they probably check your tire pressure and top it off. But if you’re in between oil changes, the pressure tends to decrease just a bit from use. During mild or hot weather we rarely notice it. And back before cars had tire pressure indicators built in, we were less likely to notice it. But with today’s cars, a big temperature drop combined with the natural leakage that happens in between service intervals is frequently enough to tip off that low pressure light.

And if one of your tires happens to have a slow leak, that just exacerbates the whole problem. But if all your tires are equally low, all you need to do is top them off. And you might be fine for the rest of the winter.

What to check when you get a low pressure tire light

tire pressure too low
This is what a slightly flat tire looks like. This one looks like it might be OK. Indeed, I’ve seen tires that look like this that have 20 pounds of pressure in them. This one was below 20 pounds and wasn’t safe to drive on.

It stinks when it happens, but the first thing you need to do when you see a low pressure tire light is stop. If you drive with a flat for any significant distance, you can shred the tire, and then you have a really big problem. Ask me how I know. If the car is hard to control, that’s a good indicator you have a flat. Put your hazard lights on and get to the side of the road. Hopefully you’re reading this before it happens, and if you remember one thing from reading this, that’s the thing to remember. If you see that light and it’s hard to control your car, pull over.

Next, assuming you’re in a place where you can do so safely, get out and inspect the tires. Tires are supposed to be round. If one of your tires isn’t, you have a flat. Change the tire or call for help if you’ve never done it before, or the traffic is too heavy for you to feel safe changing it yourself.

If it just got cold, what’s more likely is that one or more of your tires is just over the edge. If you have a tire pressure gauge, check the pressure. The correct tire pressure is always on a sticker inside your driver side door. Look for the sticker. Your tire manufacturer and your car manufacturer should agree. That was what got Ford and Firestone into trouble in mid-2000. If all your tires are a few pounds low, it’s probably just the weather. Even if one’s a little more low than the others but they’re all low, it may still be the weather.

How low is too low when it comes to tire pressure?

If your tires have 20 pounds of pressure in them or more, you can safely drive to the nearest gas station to add air. A tire can look slightly flat and still have 20 pounds in it. Slightly. That’s important. If you remember two things from reading this, the second thing to remember is 20 pounds is the minimum safe pressure to drive on. Safety first. (If you don’t remember that, just remember to Google for “tire pressure low when cold” next time this happens. This blog post will be here.)

Inspecting without a tire pressure gauge

tire pressure low in cold weather
Don’t bet on a tire that looks like this being safe to drive on. I’ve had tires that looked less flat than this have less than 20 pounds in them.

If you don’t have a tire pressure gauge, a visual inspection is better than nothing. If none of the tires look flat, you can probably safely drive to the nearest gas station. Get a tire gauge at the gas station. You should always have a tire gauge in the car. Keep it in the same place. If you’re like me, you need it at least twice a year. Yes, it took me a few years to learn that. Don’t judge. A car enthusiast may sniff at a $5 pencil tire gauge, but those are good enough for these purposes.

A tire can have a very slightly flat appearance and still be OK to drive on. The tire on the right isn’t completely flat, but I’d call that more than slightly flat. If a tire looks that far gone, I would check the pressure in it. Make sure it has 20 pounds of pressure. Note how much pressure it has. Then check the other three tires, then come back and check the flat tire again and make sure it still has 20 pounds in it. If it lost any significant pressure in the time it took to check the other three, it’s a good thing you didn’t drive on it.

Don’t forget your spare

If your vehicle has a full-size spare tire, the spare probably has a sensor in it. If it’s low, your low pressure light will go on, even if all your other tires are fine. Obviously that’s less of a problem than the tires you’re using being low. But you need to check that spare and air it up from time to time too. The last time the spare on my Toyota RAV4 went low, I couldn’t get the cover off to check it. I had my mechanic try to get the cover off when I had the vehicle in for routine maintenance. He oiled it so it would come off more easily. Once he removed the cover, he found the tire was at 19 pounds.

Topping off your tires

The days of free air at gas stations are largely vanishing. The ones near me charge 75 cents to fill up. And 75 cents usually buys enough time to add 5-7 pounds to all four tires. Remove the caps before you start filling up. I’ve made that mistake before, and what’s even more fun is when I make that mistake and don’t have any more quarters. I try to take $1.50 in quarters with me to be on the safe side. If I were a good scout, I’d keep $1.50 in quarters in the car at all times. But let’s not talk about how I’m doing on that.

The air compressors that cost money generally let you set the desired tire pressure. Then you press the nozzle onto the tire valve and it fills up and beeps at you. Watch the indicator on the machine to make sure you’re actually filling the tire. Sometimes you’re just off, and the air goes into the atmosphere instead. You should be able to hear it, but until you’re used to it, it can be hard to know just from the sound.

Even if I have a slow leak, topping it off is usually enough to either get me home or to a place that can patch a tire.

Filling your tires at home

When I was in high school, I knew people who had their own air compressors in their garage. Cars were their life, so it made sense. I’m not a car enthusiast. But hear me out on that air compressor thing. Home Depot sells a tire inflator under the Ryobi brand that uses cordless power tool batteries for power. At $40, it’s a bit of a luxury. I’m not sure I recommend it if you don’t already own another battery-operated Ryobi tool. But I do. So I snagged one on sale.

These aren’t designed to completely inflate a tire. But to add a routine 3-5 pounds of pressure to a tire to make a light go off, they’re fine. And it’s small and battery operated, so you can bring it with you on a trip.

If you don’t own any Ryobi tools, Milwaukee and DeWalt have them in their lines too. They’re more expensive and bigger, but also up to the task of inflating a tire even when it’s really low.

Can you overfill your tires?

It’s possible to overfill your tires, but gas station air compressors usually beep at you before you do. When the temperature is cold, it might be tempting to overfill your tires to increase pressure to compensate. If all your tires are equally low, don’t do that. Just top them off to the manufacturer-recommended level.

If one tire is significantly lower than the others, which suggests it might have a slow leak, I would be tempted to add a couple more pounds, to try to get longer out of the tire. But not a lot.

Years ago, a well-meaning relative suggested you could get better gas mileage by overinflating your tires by a couple of pounds. I did it during the spring, when the weather was mild. I took the car in for service in July or August, when it was hot. Thanks to the heat, my tire pressure was high. The mechanic warned me about that, and let some air out of my tires for safety. That was in 1998.

So I don’t recommend going over. Sure, in theory, if I top my tires off to 35 pounds when it’s 32 degrees and the tires hold all that air perfectly until July when it’s 100 degrees, the tires will be high. In practice, the tires lose some air in that six months and I haven’t had a problem. Except for 1998 when I did it wrong, that is.

Air it up properly and enjoy the better gas mileage.

Intermittent tire pressure lights

One time, during icy weather, my tire pressure light went on and off intermittently. One minute it was fine, then the light came on. And the light might stay on for five minutes, then go off again.

I did pull over and check the tires. I don’t take chances with my family’s safety. Of course it happened while traveling for Christmas. I filled the tires, and the light stayed intermittent even after that, once we got back on the road. We didn’t have the problem again that winter.

Maybe the next time we travel hundreds of miles for the holidays, I’ll remember to fill the tires the day before we leave. Starting out right at the manufacturer recommended 35 PSI might or might not have prevented that scare, but I would have known where we started from, at least.

Being really prepared for low tire pressure in low temps

If I wanted to have a really good year, here’s what I’d do. Where I live, the first freeze tends to happen sometime in mid November. So on a cool (not yet cold) day, I’d check my tire pressure and top it off in advance. The first day of subzero temperatures tends to happen in January. So sometime in late December, I’d check them again, and probably top the tires off again. That way, when that big temperature drop comes and steals away five pounds of pressure, I’d still be within safe margins.

If you need incentive to check ahead of time, air compressors often don’t work in subzero temperatures.

I would also keep a tire pressure gauge in each car, and a spare in the big toolbox in the garage. I might stash another spare in my smaller toolbox.

I’m not saying all of this is what I do. I am saying it would be a good idea. Since I don’t do all of these things, I make a couple of unplanned limps to the gas station most years.

Are road hazard warranties worth it?

So, all this talk about low tire pressure and flat tires brings up a question. Is road hazard insurance or a road hazard warranty worth it? I’ve gone back and forth. For the typical twice-a-year low tire pressure due to cold weather, you don’t need the road hazard warranty. It’s a better use of your time to go to the gas station and fill the tires yourself, because it takes two minutes and costs 75 cents.

Road hazard warranties will fix slow leaks for you, usually. My success in using them is about 50 percent, though. If the warranty is included and the tire price is competitive, go for it. If the warranty costs a few dollars, go for it. But when the warranty costs as much as another tire, it’s too much. Also be sure to ask who will honor the warranty besides them. The warranty does you no good if you buy it at a local shop and you get a flat 300 miles away and can’t use it.

What about roadside assistance?

The main reason I have roadside assistance is because of tire or battery problems. There was a time in my life that I couldn’t afford it, but it’s worth the $50-$75 a year if you can find a way to afford it. I’ve used a couple. I prefer AAA, because they don’t hassle me. As long as my membership is up to date, we’re good. I went with one through my credit card company, thinking I wouldn’t need a membership card since I have the credit card. That was wrong. They never sent me the card, and when I needed them, it took a couple of hours’ worth of phone calls to prove my membership.

I might or might not have written something rude on my check when I made my next few payments.

I also find the people AAA sends tend to be more interested in helping me than in upselling me. Unlike the guy the credit card company finally sent after two hours of phone calls.

I know how to change a tire, but there are times when you can’t safely change the tire yourself. That’s when it’s good to be able to call for help.

Even if you aren’t a AAA member, make sure your regular auto mechanic is AAA recommended. If yours isn’t, find one who is. There’s a difference.

One more thing, if you got this far

I don’t ask this very often, but if you got this far and found it helpful, please share it on social media. This is something that happens to everyone at one point or another, and it could save a serious injury. Or, more likely, save somebody some stress. And we can all use a little less stress, but especially right now. Thank you.

2 thoughts on “Tire pressure low when cold? Here’s why

  • November 24, 2020 at 8:03 am
    Permalink

    Just went through the cold-snap top-off a week or two ago. In west Michigan the two gas station air compressors I looked at were both credit card-only.

    It looks like we’re coming to a place where, not only should you keep $1.50 worth of quarters in your car, but now, if you were the kind of person to do without credit cards, or not carry them with you, you might need to go get one for safety. (I’ve never tried a debit card, or a prepaid Visa gift card…)

    • November 24, 2020 at 9:47 am
      Permalink

      +1 to carrying a credit card for safety. Ideally a Master Card or Visa since Card-only air compressors are a new one on me, but you never know when you’ll be on the road and face something unexpected that costs more cash than you’re carrying. It’s rare but it happens. I know there are people on the radio who say not to have a credit card, but it is possible to keep a credit card for emergencies and not abuse it. Radio Dude may think you and I don’t have enough discipline for that, but he’s probably projecting. That’s another topic of course.

Comments are closed.

%d bloggers like this: