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Discussion Starter · #1 ·
I checked my tire pressure today for the first time in a few months. (okay okay, I will do better in the future)
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All five tires were down significantly.
Is that because they were filled when it was 40 degrees warmer than it is now, or are they all leaking?
I am guessing the tire pressure monitoring didn't catch this because it looks for one tire different from the other 3, so if they are all down it thinks all is well. Is that correct?
Obviously there is no excuse for this on the normal tires, but if I fill the spare to 60psi when it is cold and then it gets hot, is it going to be okay with the overpressure (as long as I don't drive on it that way)?
Why in heck do they put the spare in with the valve underneath? (I suppose that is like asking why the center console is so crappy)

I actually checked my tire pressure because I read that AAA will not change a tire unless the spare is fully inflated. That seems silly, won't they have a compressor to fill it if it is low?
 

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Probably a bit of both - seepage and temperature. They put the spare upside down because on some models they put a speaker or tools in the middle.
 

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How much is "significantly down"? The rule of thumb is that pressure changes about 1 psi for every 10°F. So that could account for 4 psi.

Unless the spare has a major defect, it will probably be fine with any reasonable pressure swing due to temperature. Maybe if you set the pressure to 60 psi during the winter at McMurdo Station and then transported the car to Death Valley in the summer, you might have a problem. But even then, I've seen tests where tires were pumped up to 400+ psi without failing.
 

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Discussion Starter · #4 ·
My tires were down to 29 from 36. 40 degrees was an exaggeration; more like 30.
I also checked my wife's CX9. They were down to 31 from 36 after 2 months.

I don't remember other cars losing pressure so quickly; but I must be wrong because there can't be anything wrong with Mazda rims. (or could there be...) I tried another gauge, but it measures the same.

If having the valve down on the CX3 is a design flaw, the CX9 has to be lowered down on a cable and the valve is on the top. I suspect that few people ever check the pressure on the spare. But she is unlikely to ever change a tire on her own, and I hope roadside assistance will have a compressor.
 

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It's also possible that the last person to inflate them didn't fully inflate them. Either they read the placard wrong or their pressure gauge was broken. The previous gauge I used for racing read 2 PSI lower than actual across the range. Fortunately, the guy next to me in grid accidentally ran it over and bought me a new one.

Tires do lose pressure over time and there are a lot of factors for how fast.
 

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I'm religious with my tire pressure checks. I usually check them every week or two on all the cars (more on the ones outside since they fluctuate more).

I have a compressor in the garage and a 5 gallon air tank so I fill up the tank and use that to add air to whichever tire needs it.

Mazda wants cx-3 pressures at 36F 33R I do 35F 32R.

Tony
 
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