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Tire pressure in cold weather
December 28, 2010 5:26 AM   Subscribe

Car's tire pressure monitor is showing big drop in pressure. Should I be concerned?

Never had a tire pressure monitor in a car before. The recent cold snap has produced an alarm notice that all my tires are now 5 lbs underinflated. Is this just a phenomenon of physics or should I react and put more air in? I assume pressure will go up when I hit the road but have I lost air pressure or not? Worried about underinflation and overinflation. What's the correct response? Thanks for help.
posted by birdwatcher to Science & Nature (13 answers total) 1 user marked this as a favorite
 
Boyle's law- make a gas cold, it takes up less volume, so the pressure drops.

Take it out on the highway, the tires'll warm up and it should be fine. If not, you can add a little more air- but not too much, or it'll be overinflated later.

FWIW, if you're schlepping through snow, some slight underinflation will work to your benefit by improving traction.
posted by jenkinsEar at 5:32 AM on December 28, 2010 [2 favorites]


It always happens to me too when it gets cold for the first time in winter. I always have to add a little air...no biggy.
posted by murrey at 5:38 AM on December 28, 2010


Same thing happened to me a few weeks ago; the owner's manual should tell you that this is normal. One sign that it is due to the temperature is if all 4 tires are down by about the same amount; if one tire is significantly different you may have a leak.

As mentioned above it will probably improve as you drive, but if you expect it to stay cold for a while you will probably need to add a little air so you won't have to look at the warning all winter. Whether driving fixes it will also depend on how far and fast you drive as well as how cold it is. (I have 3 cars with TPMS so have a bit of experience with it).
posted by TedW at 5:52 AM on December 28, 2010


Seconding Boyle's Law.
posted by dougrayrankin at 6:36 AM on December 28, 2010


This just happened with my Kia Soul. I called my dealer and they recommended inflating to 38 psi and then checking when the weather gets warmer and releasing air if need be. It made my light go out and stay out.
posted by InsanePenguin at 6:55 AM on December 28, 2010


Not a problem, but you should air them back up. Tires work on pressure of air, not volume. And the tire pressures listed are cold pressures- before driving. As long as the pressure monitor shows a correct pressure before you start driving, ignore the pressure if it goes up while driving.

Low air pressure is bad for gas mileage. And if there is a REALLY cold snap one night, one or more of them can go flat.
posted by gjc at 6:58 AM on December 28, 2010


Some tire places can fill your tires with nitrogen instead of air. The cold air doesn't have as great an impact in reducing the volume of nitrogen. Supposedly the tire holds its pressure better with nitrogen than with air. My local tire joint does it for a buck a tire, with free top offs as needed.

Downside is you can't top them off on your own if they get low. Or, well, you can add regular air if you're in a pinch. But then the tire would need to be delflated and refilled with nitrogen again at some point (for another buck a tire) - if you wanted to stick with that. Though the idea is that, with nitrogen, one shouldn't need to add any to the tire like one would with air.
posted by thatguyjeff at 7:14 AM on December 28, 2010


Why not go out and get a little pressure gauge thingy for a few bucks? That way you can check it manually.

My car keeps telling me that my coolant level is low, but in reality, an inspection of the level upon opening the hood indicates no such drop in coolant.

Nthing all the folks above, but why not verify it with a pressure gauge?
posted by Danf at 7:40 AM on December 28, 2010


gjc is absolutely right, and in addition, low air pressure stresses the tires' sidewalls, and should you load up your car with passengers, an under-inflated tire could fail, which can be exciting for all.

OTOH, 'thatguyjeff' is misinformed. Boyles law works the same on pure nitrogen as it does on 'air,', which is, of course, almost 80% nitrogen. The theory of using pure N2 for inflating ties is that it's a bigger molecule than 20% o2 in air, and less likely to leak, but the whole thing's a scam by the tire resellers.
posted by mojohand at 7:48 AM on December 28, 2010


Some tire places can fill your tires with nitrogen instead of air. The cold air doesn't have as great an impact in reducing the volume of nitrogen. Supposedly the tire holds its pressure better with nitrogen than with air. My local tire joint does it for a buck a tire, with free top offs as needed.

FWIW, the Earth's atmosphere at sea level is 80% nitrogen already.
posted by jenkinsEar at 7:54 AM on December 28, 2010


FWIW, the Earth's atmosphere at sea level is 80% nitrogen already.

That last 20% can make a difference in cold climates. There is something about the lack of moisture that helps too. In other words, you could get close to the same effect by using air form the atmosphere that somehow had all the moisture removed from it. I suppose it could be done but it is probably easier to just get your tires filled with 100% nitrogen. I was just reading about this in the latest issue of "Road & Track" magazine.
posted by VTX at 9:51 AM on December 28, 2010


So, drove from Cape Cod up to Boston and back (about 150 miles round trip) but air pressure only got to 29 psi. Cold pressure is supposed to be 30 psi so I guess I need some air and I guess driving will only add about 4 psi. Thanks for the physics lessons and advice.
posted by birdwatcher at 11:32 AM on December 28, 2010


Same problem here, my first year with dashboard tire pressure gauge. It said my right rear tire was down to 24psi, and so I put some air in and now when it warms up it reads 37.

I don't bother with the nitrogen anymore. If I happen to be at the dealership (where it's free since I bought the car there) then sure, they can check the pressure and add some, but if I need air in my tire I'm not going all the way out there to do it... I just put air in it.
posted by IndigoRain at 9:40 PM on December 28, 2010


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