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30 degree F swings should give about 6% change (approx 30/500) in pressure. So at 40 psig (55 psia), that is only a 3 psi swing.

N2 obeys the ideal gas law just as dutifully as the other gases in air do. The difference is that air pumped in from an air compressor will typically have some moisture in it. The temperature-variable evaporation/condensation of that moisture between liquid water and gaseous water vapor is the thing that affects pressure. N2 in tires is typically dried (all H2O removed) as part of the procedure that extracts it from the air. Dry air would be just as good at resisting thermal variability.

But (as I read it) the main surpising point of the OP is that she loses pressure when temp goes down 30 degrees. And she loses pressure when temp goes up 30 degrees. And the other very weird thing is that it is the same on all 4 tires, making it less likely to be a bad valve, bead seal, puncture, etc.

So I'm really stumped on this one. Hope the info I provided above is helpful.
I would try a different pressure gauge before I spent any money on other solutions. Keep it inside at a constant temperature before using it. It's a long shot, but this would eliminate one potential source of the puzzle. And - don't just assume the pressure is decreasing because of a temperature change. You have a leak in all four tires, or someone is letting some air out when you are looking elsewhere. The laws of physics reign supreme - matter (such as the air in your tires) can neither be created nor destroyed.. Note - the lower the tire pressure, the slower the leak-down rate.

It is almost beyond any reasonable explanation why all four tires would leak down evenly unless it was maliciously caused, unless all four have a similar nail puncture or say, some cord or wire trapped in each tire bead. Once I rode over a board with a very small tack in it. I found small, similar punctures in two of four tires. Maybe a soap solution sprayed on the tires will find the leaks. Maybe someone was up to some mischief with a hypodermic needle?

Submerging one tire in a tub of water will find the smallest leak. Finding one leak might help you find the others.

Good luck.
 

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I had this problem on mine. I use to put it at the recommended psi but, now I've been running the psi higher and its actually been staying where it should be. I use to have to fill it up once a week because they would get down. I filled them up to 40psi and it seems to hold better and drip better also. I dont have that outer edge wear either like I had on my other set if tires running them at the recommended 34

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I agree that tires wear better at higher pressures. Most tires are replaced with plenty of center tread and less tread at the edges, in particular, the outside edges. For many years I have been using as a starting point about 6-8 psi over the door jamb pressure, then I adjust the pressure by a couple of psi to maintain even wear. The result is generally even tire wear, longer tire wear - and lower cost. Outside edge wear can otherwise be reduced or eliminated by a camber adjustment. With Hondas and many other vehicles, camber is not adjustable until a special eccentric bolt is used on the strut to make this adjustment (RockAuto has them). On one of my Hondas I simply enlarged the lower bolt holes in the struts and spread out the wheels at the bottom when tightening the bolts to counter excessive outside edge wear.

The manufacturers recommended pressure is a minimum pressure for the maximum rated load, or an arbitrary minimum pressure at which the ride is optimized for comfort (higher pressure gives a slightly firmer ride). The max pressure is embossed on the sidewall. Seldom are tires run at that maximum pressure.
 

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My Uber Tire Gauge got checked by NTB recently, it was 5 pounds low. So I've been running my tires a lot lower than I should have been for a while, doh! Eliminate all possibilities!
 

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Having problems with my new tires on my 2015 Ody. Since I replaced the original tires with Hankook everytime there is a 30 degree change in outdoor temperature either way (which is often in north MO) I lose pressure. It is so frustrating to have to put air in the tires when the weather is miserable, too hot or too cold!
What can I do to stop this?
Since the tire contains an enclosed quantity of air in a virtually invariable volume, every time the temperature changes the tyre inflation pressure will change too (it's known as Boyle's Law, PV/T is a constant, and in this instance we can consider V constant too). So pressure increases with increasing temperature and vice-versa. If pressure decreases with increasing temperature, you've got a leak somewhere!
 

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I agree that tires wear better at higher pressures. Most tires are replaced with plenty of center tread and less tread at the edges, in particular, the outside edges. For many years I have been using as a starting point about 6-8 psi over the door jamb pressure, then I adjust the pressure by a couple of psi to maintain even wear. The result is generally even tire wear, longer tire wear - and lower cost. Outside edge wear can otherwise be reduced or eliminated by a camber adjustment. With Hondas and many other vehicles, camber is not adjustable until a special eccentric bolt is used on the strut to make this adjustment (RockAuto has them). On one of my Hondas I simply enlarged the lower bolt holes in the struts and spread out the wheels at the bottom when tightening the bolts to counter excessive outside edge wear.

The manufacturers recommended pressure is a minimum pressure for the maximum rated load, or an arbitrary minimum pressure at which the ride is optimized for comfort (higher pressure gives a slightly firmer ride). The max pressure is embossed on the sidewall. Seldom are tires run at that maximum pressure.
The manufacturer's recommended inflation pressure is based on the assumption that you check/inflate when the tires are cold. If you drive somewhere to find an airline then the tires will have warmed by the time you get there. In this case, over inflation relative to the recommendation is wise, normally 3 to 5 psi is sufficient depending upon the ambient temperature, less if cool, more if hot.
 

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I have to admit I don't check my tire pressure unless the tire warning light comes on. That has been happening with the temperature change so that is the only information I have. I am 125 miles from the dealer but am about due for an oil change, got the tires from them just over a year ago. Will be having them check this out then I guess. Will keep everyone updated.
Did you find/solve the problem?
We have to watch our tires' pressures, what a pain to put air in with below freezing temps! Our mechanic said it's our 2003's aluminum alloy wheels. Same thing used to happen to our '96 Accord. He said the bead was not able to stay completely sealed bc of corrosion of the AL alloy. ('tho your 2015 wheels are likely different material).
 
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