Help me understand cold tire pressure

[rcroane]

Like many of you, we've had a real cold snap recently here in Virginia. I've always known that tires lose pressure when the temperature drops and I've read that I should add air when this occurs.

We've warmed up a bit here and I now see that my air pressure is too high in all of my cars. Am I now supposed to deflate them to the correct pressure? This process seems like it could be a daily occurence as temps fluctuate!

Sorry if this is a dumb question.

Thanks.
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'65 Sunroof Bug

[raygreenwood]

Actually....in relatively normal weather.....you should not add pressure to bring your cold pressure up......unless your driving length of time, ambient temps and speeds....require it.

For example: I am using my 2012 Golf for this example. Fronts with my tires should be at 35 and rears at 34.

At 7:00am.....at 40°F outside.....the tire pressures are at 32 psi front and 31 psi rear.
NOTE: its going to warm up to close to 60°F outside.

In this case...I leave the tire pressure alone. Between heat generated by driving and ambient heat going up.....by lunch time.....my tire pressures are at 35/34 again and sometimes 1 psi higher.

Next case:

Its full winter now. At 7:00 its 8°F. My front tire pressure is now 30 psi and my rears are at about 29 psi. It not going to get any warmer than 20°-25°F today.

But.....the first time this happens....do not add pressure....yet. Drive the car, wait for the h8gh temperature outside....and check them.
In this case....the pressure came up only 2 psi. I needed to add 3 psi front and rear to get the pressure where it needs to be.....and my car with my tires....handles oddly when its low or high.
Further checking over time has shown me....with my tires and average driving pattern...that this same low temperature pattern holds up.....to about 35° outside temp. Your results may vary.

So from historical checks..... I know that as long as we are in this temp range.....I have to keep 3 psi extra in the tires.
And...when constant weather warms up over about 35°F....I am going to have to let 2-3 psi out.

The thing that throws this off.....is that in when I am in the cold weather, extra 3 psi cycle...... whenever I go on a long distance highway speed drive.....like driving from Des Moines to Kansas city....3 hours at 75 mph.......like clockwork.....within about a 10 mile radius each time....about 2.5 hours into the drive.....the tires heat up enough regardless of ambient temps.....that they are now right at 2 psi too high....and.....I know this because the tire pressure light and alarm goes off.....and I stop within ahout 10 minutes at the SAME rest area to check the air just in case. 2 psi higher....while I can feel a slight difference....is fine.....so I dont adjust it....because in an hour or two when I get where Im going....I am going to be back at street speeds and normal temperature and pressure.
Ray

[Cusser]

If you convert the current and previous temperatures to degrees Kelvin, then you could calculate the pressure change due to temperature.

Like (measured pressure x Kelvin temperature when measured)/"other" temperature in degrees Kelvin. Then convert to Celsius or Fahrenheit, whatever.
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[raygreenwood]

[rcroane]

Thanks, guys. This is very helpful.
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'65 Sunroof Bug

[gt1953]

Yesterday mid morning aired up all of the tires...Some took almost ten lbs. Three VW's a pickup and Subaru.
No one mention elevation change. Ok so go from 6500 feet to 1000 feet. What does the pressure do??? I know I know. When you go down in elevation so does the pressure. When you go up so does the pressure.
Not sure why but I bet a scientist type knows.
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Numbers Matching VW's are getting harder to find. Source out the most Stock vehicle and keep that way. You will be glad you did.

72 type 1
72 Squareback
({59 Euro bug, 62, 63, 67, 68, 69, 73 type ones 68 & 69 type two, 68 Ghia all sold})

[iowegian]

[Pruneman99]

Nitrogen fluctuates less with changes in temp and can lessen the psi swings. It is used in aircraft and even in new cars. Is it worth it? IDK. Maybe if higher performance is required or you notice handling issues with a slightly low, or high psi.

[gt1953]

Purchase a bag of chips or something like it at a low elevation...then travel up the mountain. Watch the bag swell up.
_________________
Volkswagen: We tune what we drive.
Numbers Matching VW's are getting harder to find. Source out the most Stock vehicle and keep that way. You will be glad you did.

72 type 1
72 Squareback
({59 Euro bug, 62, 63, 67, 68, 69, 73 type ones 68 & 69 type two, 68 Ghia all sold})

[EverettB]

[Cusser]

[raygreenwood]

[Cusser]

[Bob Loblaw]

[EverettB]

[Bob Loblaw]

[sb001]

[Bruce]

[heimlich]

Drive the car until the tires can warm up from road friction. Then add your air making sure not to overinflate. The heat from driving on the road will heat the air inside the tires.
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[doublecanister]

I always check mine on the weekend (usually cause I have the time) the car has sat overnight so I have cold tires.

Midday/Noon-ish is "usually' when I get around to it.

But as you say in Virginia, you pretty much have to do this monthly esp in real cold weather sub 20degrees.

One thing to note: your Schrader valve in your air valve stem, I've had a few of these leak on one of my vehicles, the valve had small bits of plastic on the cork/rubber type surface causing a very tiny leak,
the old (spit on yer finger and check the valve for leak trick) didn't even notice it.

I try to keep a few spares handy and the wrench to check/clean/tighten or replace them.

Hard to believe that's the only thing keeping air in the tires!
Really been a pain on my 73 Thing's washer and spare tire pressure. I usually end up with a flat spare, think I have a slight leak in the rubber hose to washer screw top... Anyways...

And if you get curious, its real easy to "shoot one across the driveway" if you take it out while the tires got pressure on it.

I always heard you should check tire pressures monthly anyways for best MPG.

T
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1973 - Thing
1966 - Mustang GT- Fastback
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