Summertime tire pressures - what counts as "cold" for setting tire pressure

[Neggytive]

Before you drive the car and add heat to the tire by friction, Heat causes pressure to go up, by way of Boyle's Combined Gas Law

That is why you get a low pressure warning when the tires you filled at 75 degrees ambient sit outside on the first cold night of fall or winter and drop to 40 degrees

Normally your pressure will go up about 3 degrees when you drive the car and get some heat in the tires.

Set the TP hot, the car cools off, the pressure drops 3 PSI or more, and we are looking at the Ford / Firestone fiasco of the 90's, which is why we have Federally mandated TPMS monitors

If you notice the TPS monitor comes on at about 2 to 3 pounds below its target pressure.

So my answer is... when the car has sat for a while, overnight is better, and has no added heat that would cause the pressure to be higher than it would at static rest

https://chem.libretexts.org/Bookshe...The_Behavior_of_Gases/14.06:_Combined_Gas_Law

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[fishinrich]

Use common sense and you'll make it through this.

 

[ChitownStang]

I have always done 32psi cold.
That’s what is recommended in your door sticker.
Always had great traction and tire wear

 

[Achlys]

Cold tire: sitting for about 8 hours in the shade. Pressure should read at the proper PSI from your driver's door panel.
Hot tire: a tire that has driven over 1 mile. No, it is not ready to drag-race; only the tire's interior temperature affects PSI.
Cold weather: based on regular air, not nitrogen, for every 10 degrees drop in ambient temperature, a tire will loose 1 PSI. So if you go a month, say in October when temps change, your PSI could be low, which could affect tire wear and traction. Nitrogen removes the majority of these issues.
For the OP, messure the tires at the outside temp, after a mile or so, they will warm up to the proper temp. Or find a retailer that uses nitrogen, and all will be well. Nitrogen does not fluctuate like regular air, hence the reason nitrogen is used in jet plane tires.

 

[MCS]

I run 32 cold in the summer
I run 32 cold in the winter when it's sitting in my garage

I have a portable air pump that I set to 32 every 6 months.

 

[LouG]

What did surprise me was the 32 spec. It seemed low, my last car had 42psi Fr and 30psi Rr for 245/35-20 tyres.
But it works

 

[Starship Enterprise]

OK I get the OP question. Set your cold tire pressure to the baseline of outside, not the car park.

Think of it this way, if you parked your car in a giant freezer, your tires would be way out of wack when you got outside on a warm day.

If you inflated your tires to 32 in the cold garage where it's 20˚ cooler than outside, (you lose 1 PSI for every 10˚ drop in temp) now your tires are overinflated by 2 PSI in the environment you are actually out driving in.

That said it's a minor thing and can be overthought....lol

 

[RLE55]

To get technical for a moment:
The rule of thumb is, for every 10 degree change in ambient temp., tire psi will change by 1-2 psi. When I was road racing, I'd check my tires every time I'd come off a session using with a good tire guage and pyrometer from Longacre Racing, never trusted cheap stuff. I knew the optimum temps for my tires (Hoosiers) and would adjust the psi accordingly.
As many have stated, go by the days ambiant temp as the "cold" reading and adjust from there.
Reminder, friction of the road causes tires to heat up and its completely normal and expected for tire pressure to rise by 4 to 6 PSI after driving for 30 minutes at highway speeds. Watch the TPMS reading on your dash, it'll tell you if theres an issue.

 

[Junkyard Dog]

I knew the optimum temps for my tires (Hoosiers) and would adjust the psi accordingly.

What was the optimum temperature? Or did you mean optimum psi? If so, what was the optimum psi?

 

[Neggytive]

The optimum PSI for a race tire depends on a few factors, but one of the biggest is you want tire patch contact

I have cars in the shop and can look at the wear pattern and tell a lot about the car, including if the tires have been over inflated (center of the tread worn out) or under inflated (edges worn, reduced tread contact in the center)

Tires also act as a spring, so there is some consideration for tire PSI so you'll see adjustments made for that.

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