Ambient Temperature for Recommended Tire Pressure

Last weekend I took the winter tires/wheels off the CX-9 and put the OEM tires/wheels back on. Since they'd been sitting for a couple months, I wanted to make sure that the OEMs have the appropriate tire pressure.

Now before I go on, let me tell you what I understand to be true re tire pressure:

• The placard in the door frame of my CX-9 recommends 34psi all around;
• This is the "cold" pressure (i.e. the pressure taken before the vehicle has been driven);
• Tire pressure will increase as I drive because of the heat of the tire itself; and
• For every +/-10* change in ambient temperature the pressure will drop or rise by ~1psi.

So my question is this - at what ambient (outside) temperature should the pressure be at the recommended 34psi?

And yes, I know I'm probably totalllly over-thinking this.

I am the same way and I agree with everything you stated. What I do is set the pressure based on the average temp for the next month or two. I always pick a number that will work should the ambient temp rise or fall by about 10 degrees. So here in NY, in December-Feb, I know the temps avg. about 25 to 30 degrees but can fall to 20 or sometimes a bit less, or can go up to about 40. So I will set my pressures to 34 on a 30 degree day knowing that sometimes they'll be down a pound or up a pound which should be just fine. So I end up adjusting my pressures every 2-3 months or so depending on the weather. The next time I set them will be on a 50 degree day as we approach April. I am a bit OCD on this and I always have each tire exactly the same, not even .5 different. My GLC has real time pressures and I make sure all 4 are always reading the same.

I know I'm probably totally over-thinking this.

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Agree,

One or two psi is not a big deal. Check, correct, don't worry. In many localities there can be a 20 difference between night and day. Or more. That alone is not the cause of problems.

The ambient temperature is whatever the outside temperature is. So in the summer, July, it may be 90* and you would set them at 34#. Then in December when it's 20* you would adjust them to 34 again. It will always fluctuate so keeping them at 34 exactly will never happen day to day.
The pressure of your tires will also change as you go from sea level to the mountains and back. When I leave Texas in October for hunting it can easily still be 90 out and at 650' elevation. I get to camp in Colorado and it can be 25 at 9000' elevation. This gets a little trickier because of both changes but not bad. I gain about 3.5# increase due to the elevation change but lose 7ish because of temp. Since I run a few psi higher than recommended I don't worry about it. But if the temps were the same he and there I could get to a point where I would have to let out some air when I get there and add some during the drive home.

Tire Rack has a lot of good info on their website regarding tire pressure. Here's a link for what they have to say about Air Pressure: When & How to Set

Personally, I'd rather be slightly overinflated than underinflated. Underinflation can cause greater heat build up within the tire and shorten its life.

It is 70F. I go 1 PSI for every 10 degree difference.