Tire pressure

37chief

Well-known Member
Location
California
I checked the pressure in a tire this morning Temp was probably around 32. Pressure was down from predvious day by 8-10 lbs. About mid day it was back up the same amount. So if I want 80 lbs in my tires, is the pressure going to increase a lot when the tires hit the road. Should I just put in 80 lbs in at mid day and not worry about it? Stan
 
Stan

Set your tire pressure at 80 with the tires "cold", meaning not driven more than a mile or so. The pressure will increase slightly when driven, maybe 4-5 pounds. That"s the way it should be done.
When the outside air temp goes back up (lets say to 60 or 70 degrees) then you need to check your pressure and adjust acordingly. Air pressure should be checked monthly.
 
Check tire PSI cold. If you are going on a trip fill cold and if you check them hot you are just making sure they are the same, if three are 90psi and another is 75 psi you know the low tire needs attention.
 
Stan. Are you saying the pressure changed by 8-10lbs. with out even driving it? Went down first and then back up? Only thing changed was the time of day? You gotta get a new tire gauge. That's where your problem is.
 
The tire guage is top of the line, no china crap. Tire was very cold in the morning, and hot in the afternoon. I will check again in the morning, as I know the exact pressure now. Stan
 

Also will change like that as Low Pressure weather areas pass your area and high pressure builds back in. What you see on your tire gauge is a pressure [b:aed806f58d][i:aed806f58d]differential[/i:aed806f58d][/b:aed806f58d] between the air in your tire and the outside air pressure.
 

Also as the air in the tire heats up, the pressure will go up. You have to anticipate the heat build-up as you drive down the road. That's why the tire's rated pressure is "cold". If you have too much pressure it the tire when cold, it will probably build up too much pressure as it heats up while going down the road and blow out.
 
Be sure to check the air pressure in your spare tire too Stan! The spare is out of sight out of mind usually.
 
10 degree temp change will cause about a 1 PSI change in tire pressure.
That is why us "northerners" put winter air (denser) in our tires each fall & summer air (not as dense) in come springtime. :>)
 
Air pressure on my truck tires only varies 1 or 2 pounds usually. They always drop slowly, but rarely gain more than a pound or two no matter how hot they get. I can count on about 5 lbs drop per month in my truck tires, a little more in the winter. I also would say try another pressure gauge. Heres a link to a test.
Untitled URL Link
 

It is not all that unusual for an inexperienced tractor pulling competitor to get disqualified because of setting his hitch height early in the morning, and then those big tires sit in the hot sun for a couple hours and they grow.
 

I run tires with heavy loads on them, so they get hot and the air inside them gets hot and expands, causing high pressures. I have noticed with the quality of tires going down these days, air will leak out easier. The air molecules are able to leak out past the tire plies with the heat and higher pressures. The heat deteriorates the tire material and the air is able to escape more and more. Nitrogen has larger molecules which stay in the tire better under pressure. But I don't use Nitrogen in my truck tires. I have noticed I have to add air more to the low-profile tires on the cars nowadays. They just don't hold enough volume of air to maintain pressure for very long. I guess that's why some shops put Nitrogen in those little tires. I think Green Slime helps seal the tires against heat deterioration and thus helps to maintain air pressure, but sure screws up those in-tire pressure sensors.
 
You need to change out all that summer air, and replace it with winter air, it's more dense, and less humid, so pressure stays more the same. If you live near a dairy, don't put in no dairy air...
 
We run two heavy haul rigs here on the shop that each have 44 tires. Sometimes grossing 140,000 or more. Tire pressure is a big thing. Just cannot let any one tire get low or things go wrong quick.
We all-ways check air before the rig leaves with the tires cold. As one poster says if the thing just sits sometimes it will loose air for no reason. Slow leaks are really hard to find.
 
The nitrogen theory is a sales gimmick. Ambient air, what is out there in nature, is 80% +/- nitrogen, & 20% other gasses & water vapor. Thing about bottled nitrogen is that the process of compressing & filling the tanks squeezes out the water. Same thing happens with air. Ever notice the water collected in compressor tanks? Water vapor expands with heat more than the other gasses. A good drier, such as used inline for paint guns, will give the same result without the major expense. Size of molecules has very little, if any, effect on leakage.
Wayne started out right, 10 deg temp change makes approx 1 psi pressure change. To stretch the point a bit, winter air does actually contain a bit less water vapor tham summer air.
Willie
 
Check your air pressure about the same temp each day adjust up or down a few lbs to make up for the temp difference. The pressure is a estimated amount it doesn't hurt to be a little off. On your car the smaller the tire the closer you should be to the amount on the door or under the hood. On big trucks with large tires they can be anything from around 85 to 110 lbs depending on how much weight you are carring. On my big trailer it's says 110 for 3100 lbs per tire. On my dump truck it is around 85 to 95 depending on the load. But it's not critical it's a estimated pressure.
Walt
 
Air them up cold. In automotive a "cold" tire is one not driven on more than three miles. Air them to the cold pressure on the tire placard and drive unless you have LT tires. In that case air them up to hold the load. Afterwards lower the pressure to the tire placard.
 
We've got 4 new NAPA tire gauges in the last year. Two of them were junk. One wouldn't read anything under 90 PSI. The other would read 30 PSI at 40, and by the time the tire hit 80, it was reading about 54.

The other two new gauges read within 2 PSI of every other gauge on the place at 80 PSI.

Because it's new and high-dollar doesn't guarantee accuracy.
 

For dry air, using the Combined Gas Law for a constant volume (assumes the volume of the tires do not change) we have: Initial pressure x Final temperature / Initial temperature = Final pressure. Where the pressure is absolute, psia (add 14.7) and temperature is absolute, degrees K (convert to centigrade and add 273). You did not state the two temperature points so we will use 80 psi at 70 degrees F and compute the pressure at 32 degrees F.

For example:

Initial pressure is; 80psi + 14.7 = 94.7 psia.
Initial temperature is; 70 F = 21.1 C + 273 = 294.1 K
Final temperture is; 32 F = 0 C + 273 = 273 K

We have:

94.7 psia x 273 K / 294.1 K = Final pressure = 87.9 psia and subtracting 14.7 psi results in 73.2 psi. The 73.2 psi is what is read on the tire pressure gage at 32 degrees F.

In conclusion:

Yes, depending on the two temperature points the tire pressure can vary significantly. A rule of thumb is 1 psi per 10 degrees as other have posted. This rule may be more accurate in real world situations, I do not know - it is certainly easier to remember. Remember, it is the temperature of the air in the tire not ambient air temperature - if the sun is shining on the tire it can be much hotter than the outside air temperature. Maybe your gage is ok after all - hope you have not thrown it away.
 

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