Filling Tires On A Cold Day

[tom538]

The temp outside today is 48 degrees. I need to check the tire pressure on the six tires on my Fleetwood Expedition and inflate them to 105 lbs if needed. Will I get lower readings than normal because of the 48 degree temp as opposed to checking them on a warm day?

[wolfe10]

CORRECT. All tire manufacturer's inflation tables are for cold. Cold is at current ambient temperature and before driving.

[desertdeals69]

Have you had your coach weighed? Sometimes the air pressure is different in the front than the rear.

[vsheetz]

I think these address the OP's question:

http://www.bridgestonetrucktires.com/publications/02v7iss1/ra8.asp

http://www.rv.net/cforum/index.cfm/fuseaction/thread/tid/26556591/srt/pa/pging/1/page/1.cfm

[andyshane]

I can't seem to find the reference, but I recall that the cold inflation value in the manufacturer's tables for your tires is based on servicing them at 60F.

In general, colder results in lower pressure at a rate of 1PSI per 10 degrees F. Let's say your chart calls for 105PSI and you're inflating the tires when it's 40F. Theoretically, you'd want to service them to 103PSI. Of course, in real life, you can hardly see 2PSI on the gage...

To recap lots of prior discussions:

• Use the real weight as the rig is normally operated to find the right pressure, never the placard weight limit inside the rig. Those weights are the maximum for which the rig is certified, and the tires may or may correspond to those presently installed.
  
• Ideally, each wheel is weighed independently, since some folks load their RVs unequally. For most of us, the truck stop scales (they give you weights per axle) are good enough.
  
• Read the inflation chart from the tire manufacturer carefully. Some use axle-end weight, others us individual wheel weight in their charts.
  
• The chart values are usually optimum generally, not minimum. Even so, jump to the next higher weight, if on the border between two values.
  
• Some people add 5PSI in case they drive after some air has bled/leaked. No harm in that, an individual choice.
  
• If you've read the chart wrong and have a lower pressure than needed, the tire will run hot, ie over 120-130F. Adding air will cool the tire's running temp (can be scanned with a $20 IR temp gage). Remember, the tires running in the shade will trend cool, the sunny side ones, warm.
  

The 105PSI value sounds a little high for the Expedition. Higher pressure (up to the DOT sidewall maximum, which is only used to inflate towable tires) should'd hurt. It will just give you a rough ride, shrink the contact patch. Since the Expedition should carry almost exactly 2/3 of its weight on the duals, front and rear tires should be loaded about the same and thus need the same pressure. This is not true of many RVs.

[wolfe10]

Andy,

I can't seem to find the reference, but I recall that the cold inflation value in the manufacturer's tables for your tires is based on servicing them at 60F is an internet hoax .

That is why you will not find this on any tire manufacturer's website.

Cold is defined by all tire manufacturers as "at current ambient temperature before driving".

[five]

Andy,

...Cold is defined by all tire manufacturers as "at current ambient temperature before driving&quot...

Right, fill them to the PSI you want where you are when you are going to start driving.

[Briarhopper]

A 20 degree difference is not really enough to be concerned, but just to be clear for those who pass by this thread.

If you inflated your tires to 105 psi at 65 degrees, then they will read lower by roughly 2 psi at 48 degrees.

If you are checking at 48 degrees early in the morning and the daytime high is going to be back to 65, then you allow that the psi will be roughly 2 psi low when you check, but no need to add air.

If your cold inflation pressure is 105 psi and you plan to be driving in 20 degree air and your coach is sitting in 20 degree air when you check, you would inflate your tires to 105 psi.

If you had a heated garage and your coach was sitting at 70 degrees and you were going to pull out into 20 degree air, you would inflate to 110.