Proper cold weather tire inflation practice?

subaru
tires
impreza
air-pressure

#1

As we all know, proper tire pressure is vital for tire life, proper handling of the vehicle, fuel milage, etc. It is also somewhat common knowledge that proper pressure is based on the cold pressure - i.e. when the tires are stabilized at ambient air pressure. In my case, my car is parked in my 60º garage overnight, and outside during the day. The outside temp is forecast to be getting down to -40 (ºF or ºC, take your pick, they are the same at that point) most of this week, so we are talking a 100º difference between the outside temperature and the temperature in my garage.

Searching the internet, I find that the rule of thumb is that tire pressure changes about 1 psi for every 10ºF change in temperature - so the pressure difference from when I leave in the morning from a 60º garage, to when I come home from work where my car has been sitting outside at -40º could be as much as 10psi - which, I would think, would be significant. Of course, the temperature right now is only about -15ºF, so not so cold.

My thinking is that given the low outside temperatures, I should be “over” inflating my tires somewhat when measuring the pressure in the warm garage, so when they cool down after sitting outside all day, I’ll still have sufficient pressure. The question, of course, is “how much”?

Using the rule of thumb and a cold temperature of -40 would say I should over inflate by 10psi - bringing the pressure in the tires up to 42/40 psi front/back (the car placard states 32/30). The maximum inflation pressure on the tires is listed as 50psi, so this should be safe, but is it best? At the current temperatures, that would still be over-inflated by at least a couple of psi, but that doesn’t sound bad to me, and definitely sounds better than being under-inflated. On the other hand, in the mornings the tires would be over-inflated by around 10psi, at least for the first part of the trip (depending on how the tire temperature changes as I drive) - could being that far over cause a problem?


#2

How about inflating them when the temp already is cold.

I probably adjust the air pressure on my tires at least 10 times during the winter months.


#3

That would mean leaving the car sitting outside at -40 until it has had enough time to cool down, and then working outside at that temp to check and inflate the tires. No thanks - part of the reason I got a nice warm garage is so that I could work on the car inside when it is cold :slight_smile:

That said, I have no problem with the concept of, as you say, adjusting the air pressure many times during the winter. The only difficulty is figuring out what to adjust them to if I am inflating them in the garage, coupled with the fact that temperature swings widely from day to day. This week is supposed to hover around -40. Saturday it is supposed to get up to +1ºF. So if I really wanted to stay on top of things, I’d be adjusting the pressure weekly, most likely, if not daily - but then, that would probably be overkill. So maybe I should just pick an in-between pressure, and re-adjust monthly?


#4

You wouldn’t have to work on the tires at -40. Park the vehicle outside in the cold for several hours, then bring it into your heated garage and check pressure right away.


#5

Although I don’t normally advocate underinflated tires, your case seems extreme enough that I’d try to set the tires for a -20 temperature as the best compromise. When it’s -40, your tires will be underinflated, but not enough to be unsafe (or to set off the TPMS, most likely). I’d say to start with 40/38 PSI in your garage and do one quick measurement of a cold tire outside in the shade, after which you can make any needed adjustments in your garage the next morning.

When you’re shopping for your next car, you might want to specifically look for one with a direct TPMS system, which would read and display the exact pressures as you’re driving. That would be interesting information in your situation.


#6

You wouldn’t have to work on the tires at -40. Park the vehicle outside in the cold for several hours, then bring it into your heated garage and check pressure right away.

Oh, right. Duh :stuck_out_tongue:. Sometimes my brain doesn’t work.


#7

Although I don’t normally advocate underinflated tires, your case seems extreme enough that I’d try to set the tires for a -20 temperature as the best compromise. When it’s -40, your tires will be underinflated, but not enough to be unsafe (or to set off the TPMS, most likely). I’d say to start with 40/38 PSI in your garage and do one quick measurement of a cold tire outside in the shade, after which you can make any needed adjustments in your garage the next morning.

Makes sense. Thanks.


#8

How about checking them right when you get home in the evening? They’ll be warmer than they would at the end of your morning commute, but cooler than when you first leave in the morning.


#9

I agree with your analysis. Or with bringing them in after several hours at -40 and immediately inflating to spec.

By the way, I always run my tires a bit overinflated compared with spec. It has served me well for 50 years. I believe that spec is oriented to comfort, not long tire wear. Not to hijack the thread…


#10

Yes, winter is a problem. When it gets cold in Minnesota, I slightly over-inflate my tires in the warm garage. Then when outside running around, I just monitor a little the tire pressure. If they look low outside, I’ll add a little more air. Really not many places get down to -40 for any extended period of time or even -15 for more than a few days. Usually we have a normal range of maybe +5 to +30. In cold weather I’d rather have a slightly over-inflated tire than one way under-inflated. I don’t think there is any reason to get over analytical about it. Just beware that temperature has an impact on tire pressure and adjust maintenance accordingly.


#11

Is this car ever parked outside for extended periods of time, like all day while at work? I am getting the sense that it is not. If that is the case, then the tires should never get down to this extreme temperature as they will stay warm from the friction of driving.

Personally I would carry a gauge and an electric pump with me. Set the tires 3 to 5 psi above the placard and then drive it. If it is parked for any significant period of time, check the tires visually before driving again. If the sidewalls are not too flat, then just drive it. If the tire appears too flat to drive, the add enough air to bring the tire up to about 10 psi below the placard value.

Having low pressure, but no too low, during extremely low temperatures will help the tires heat up as you drive them. The heat will increase the pressure and also increase traction.


#12

Keith, I’m not sure that is a good practice. I’d set the tires to recommended pressure on a day that is about as cold as it gets. Or overinflate them based on the 1 PSI per 10ºF rule if it is warmer.

Over is much better than under.


#13

From my original post:

So yes, it is outside for extended periods of time, like all day while at work :slight_smile:


#14

I am with Bing, this just does not sound like good advice. It seems it would take a lot of driving in cold weather to increase the psi 10 lbs.


#15

Yeah, that’s what I was thinking. My main reason for posting is to see if there were any potential pitfalls with that approach that I wasn’t thinking of. That said, it does seem to make sense to do something like check before I head home from work (or some other time after the tires are cold), and adjust accordingly to really narrow it in.


#16

OK, I missed the outdoors all day part. But in my defense, the next two replies led me astray. So now my question is, what are you doing now? If you are inflating them in the garage, do they appear flat after sitting out all day? I would not think that checking my tire pressure would be a task too difficult to do in cold weather, but then our lowest temp this year was +13F.

In extreme cold weather, over pressure is not a good thing because it keeps the tires from warming up. 10 psi low on a modern winter tire in extreme cold weather is not going to hurt the tires. I would go by a visual inspection, if they look OK, then they are good to go.

BTW, I am a little OCD on tire pressures, when I check them, I do use a non contact IR thermometer and use 68F as the cold temp. Then I adjust the pressure by 0.1 psi for every degree off from that and I use a calibrated tire gauge that measures increments of 0.1 psi. But then I don’t see -40 here. Not real sure what I’d do in that case but I do know I would not pump them up to the placard pressure at -40.

Once I set my tire pressure, about every 3 months, I do not mess with them. I just check them visually periodically to make sure they don’t appear low.


#17

It is standard procedure to compensate for outside temperatures when filling tires in a heated shop. Inflate your tires to compensate for the conditions they will be operating in, they will not stay warm for long in those temperatures and the pressure will drop.

This bulletin provides instructions and a temperature chart to assist in setting the tires to the proper pressure;

The pressure chart doesn’t show a wide enough temperature range but gives a general idea of how to compensate.


#18

Going from 60F to -40F will actually reduce 32 psi to 23 psi, a 9 psi drop in pressure. That rule of thumb is an oversimplified formula that’s “close enough” as long as you are dealing with normal temperatures and typical car tire inflation pressures.
For example, if you inflate your 10 speed bicycle tire to 120 psi at 60 F, it will drop to 94 psi, a loss of 26 psi. at -40. The oversimplified rule of thumb no longer works because 120 psi is out of the domain of typical car tire pressures, even though it’s common with racing bicycle tires.

Driving from a high elevation to sea level will also change your tire pressure, so if you live in Salt Lake City and you decide to drive to Death Valley, you might want to check your tire pressure along the way.


#19

One thing about putting air in the tires when it is cold out (I mean really cold, like below zero), is that the air can contain moisture and will freeze and hold the valve open. The result is a flat tire. For the last 40 years I have never put air in my tires in the cold unless it was absolutely necessary. Then you make sure you over inflate slightly and then push the valve down to release some air. Then of course put the valve cap on. I have my own compressor so very rarely would need to use the gas station units but still it is something to keep in mind while you are adjusting pressure due to temps.


#20

One day in November the TPMS light came on so I put more air in the tires. I won’t check them unless the light comes on again. Maybe when the winter tires get swapped out. Then I plan to check them in September or before I take a trip. Life should be that easy.