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Old 01-01-2013, 04:43 PM   #1 (permalink)
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Tire pressure in cold weather

When temperature dropped below 40, I noticed a drop in PSI from 32 to 28 (from trip computer readout). As expected in cold season, I drove to gas station and filled each tire to 32 PSI using a manual gauge to check. After tires cooled off next morning, the trip computer was showing 29 PSI and checked with gauge showed 32 PSI. After driving on the highway for 30 min, the pressure on all 4 tires was showing 32 PSI on TC.

Which readout should I follow? TPMS warning light never came on, all 4 tires showing normal pressure. Just curious. Thanks
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Old 01-01-2013, 07:06 PM   #2 (permalink)
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What kind of gauge do you have? If it's the pencil type, those aren't known for accuracy. My understanding is that the mechanical dial type (with a range such that your normal pressure is roughly halfway up the scale) is most likely to be accurate. I'm partial to Accu-Gage myself. (All three of mine match perfectly.) Can you have some friends and family members use their gauges and compare the results?

By the way, unless your gas station is really close, your tires were no longer cold when you got there, so you should have gone higher than 32 PSI there. In the future, you should do one of two things. Either make note in the morning of how many PSI need to be added and then add that amount, or fill the tires to 36 PSI at the gas station and then let out the extra air the next morning.

While you're sorting this out, I'd suggest going by the TPMS, because it's showing the lower number. Overinflation is safer than underinflation.

I'm not surprised that the warning light didn't come on. Those are usually designed to come on at a lower pressure, when there's an immediate safety issue.

Did you remember to check your spare?
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Old 01-01-2013, 07:49 PM   #3 (permalink)
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I use a dial type gauge. It has matched my X and my other two cars that had tpms.

As mentioned, once you start driving, the tires are no longer 'cold'.
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Old 01-01-2013, 10:26 PM   #4 (permalink)
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You should measure your tire pressure "cold", meaning when the car has been sitting for at least several hours (tire pressures increase by several psi when the car is driven, due to the tires warming up), at the typical outside temperatures for that time of year (e.g. NOT in a heated garage), and NOT sitting in the sun (which can also increase pressure). Lion provides good advice above for how to get around this if this is a problem.

Note, the TPMS in my 2004 MDX only triggers at 75 percent of normal pressure, so the pressure would have to go down to 24 psi before the warning light goes on.
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Old 01-01-2013, 11:46 PM   #5 (permalink)
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Thanks. Some useful info. Will process and adjust PSI.
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Old 01-02-2013, 02:45 PM   #6 (permalink)
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Every winter I fill to 35psi cold and find myself doing it again every 4 to 5 weeks because I tend to lose a lot of pressure during the cold months.
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Old 01-02-2013, 03:01 PM   #7 (permalink)
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Originally Posted by Dominik-X View Post
Every winter I fill to 35psi cold and find myself doing it again every 4 to 5 weeks because I tend to lose a lot of pressure during the cold months.
You shouldn't lose a lot of pressure during winter. However, it's normal to lose pressure during fall (autumn), because that's when temperatures drop. The Tire Rack estimates that, without any other changes, the pressure in a tire will drop by 1 psi for every 10 degrees F drop in temperature. So, for example, if you set the pressure to the recommended 32 psi on a warmer day when the car has been sitting at a temperature of 70F, and then it gets cold the next day and you check it when the temp is 30F, you'll probably find that it's down to 28 psi or so. This is why tires lose pressure in autumn, as temperatures transition from warm to cold, but not during winter, when temperatures stay cold. They also tend to gain pressure in spring, as temperatures warm up.

Regardless of temperature changes, it's a good idea to check your tire pressures at least once a month. Tires tend to lose 0.5 psi per month just from sitting around (on OR off the car).
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Old 01-02-2013, 03:30 PM   #8 (permalink)
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Originally Posted by Dominik-X View Post
Every winter I fill to 35psi cold and find myself doing it again every 4 to 5 weeks because I tend to lose a lot of pressure during the cold months.
How much is "a lot"? If it really is more than normal, have you sprayed soapy water on the tire to look for leaks, especially around the valve stem and the bead?

As nsxtasy says, you shouldn't lose any more during cold weather than during warm weather.
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Old 01-02-2013, 03:45 PM   #9 (permalink)
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We had about 20 degrees temp drop in the last couple of weeks and I lost 2 psi since the last time I filled, so your assesment is correct.
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Old 01-02-2013, 04:19 PM   #10 (permalink)
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We had about 20 degrees temp drop in the last couple of weeks and I lost 2 psi since the last time I filled, so your assesment is correct.
Yup - that would be due to the drop in temperature.

Here's what I do to avoid worrying about short-term changes in temperature (and two weeks is short term). I set the pressure based on the normal temperature for the time of year, so that it doesn't vary within a couple of weeks. For example, during the day here this time of year, it's normally around 30F. So I'll set the pressure to 32 psi on a day when it's 30F. If it's not 30F, I'll either (a) set it higher or lower to account for the temperature difference (for example, if it's 50F, I'll set it to 34 psi, knowing that it will drop to 32 psi when it's 30F outside), or (b) set it several psi higher than I would expect, then let out enough air when it's 30F so that it's exactly 32 psi. Same kind of stuff lion was talking about in his post above.

Granted, if I set it so that it's the 32 psi I want on a day that's 30F, the pressure will vary from that whenever the weather is warmer or colder than normal. But that's always going to be the case. I don't adjust it just because we get unusually warm or cold weather on any particular day.

Hope that makes sense!
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