My JK and wifes car just dropped 6PSI in the last few weeks due to the change in temp. All the tires were the same: 29PSI all round when cold after the seasonal temp change as we go into the autumn. The TPMS was on for both vehicles which alerted me to the change. I air them up at home with a small compressor i bought from BJ's for $50 last year. FYI. Cheers.

 

You make a good point. I have to remember who is reading this.

I know most advise checking them cold, but that is never a truly accurate method. Tires will tend to hold operating temps over a wider range of outside air temps. I have a neat conversion chart that compares cold pressure to operating pressure over a wide range of temperatures.

I'll check the pressures hot, and adjust them. For example: At 40 degrees, 32 PSI will produce 37 PSI when at operating temperature of 90 degrees, 39 PSI at an operating temperature of 110 degrees. For best tire wear and handling, I set them hot. This is my years of road racing influencing me.

For most folks, setting pressures when cold is probably best. Not the most accurate over a temperature range, but certainly easy for the average driver.

 

I'm not sure how to take the "I have to remember who is reading this" remark.
Anyway, Are you talking about best tire wear and handling on a race car or a Jeep that is driven nothing like a race car? I'm just speaking from years of experience from not giving the tire companies my money because I get maximum tread life out of my tires (barring any unfortunate unfixable puncture) with proper inflation and rotation schedules.
Now I'm not saying you're wrong, it's hard to convey actual feelings on a subject over a forum setting, but I just really don't understand applying the ideas of tire pressure management of road racing over daily driving in a non-road racing vehicle. You're right about it not being for the average driver though, it overcomplicates the simplest thing you can do.
I would be interested in seeing that conversion chart you speak of though, I feel one is never too old to learn something new.

 

Yeah, sorry about that vague remark. I write to a few road racer forums as well.

My charts are in a racing tire selection guide, but I was able to generate a simple chart using online resources that should be useful.

 

Widewing, racer PSI changes are because you HAVE to change them when hot because you're in a race and want the tires hot for fiction from the get go and you have to factor that in for the PSI...

But that isn't to say that a cold PSI is a bad/poor baseline, in my opinion.

 

There's many reasons to adjust your tire pressures, and I have run different pressures on each tire. Race tires get much hotter than street tires. Thus, you may start out with cold pressures 8 to 10 PSI lower than when the tires are fully up to temperature. Pressures aren't only about adhesion. You have to look at turn in, understeer, oversteer and getting power down without wheelspin. You have different suspension (shocks, springs and sway bars) setups for different road courses and you also have dry and wet setups as well. Invariably, you will check tire temps across the tire during practice and adjust your pressures to optimize equal heating across the tire. You cannot accomplish this cold. Since I own a tire pyrometer, I check my tires for uniform temperature across the tread using the pyrometer and I do it after getting them up to operating temperature. This tool will not be found in many garages. Therefore, adjusting pressure cold is the obvious method recommended by manufacturers. It is not, however, the most accurate way to determine ideal pressure for wear or performance. Moreover, the hot check using a pyrometer is superior to the common chalk test cold.

Just remember that the above only applies to driving on paved roads.

 

Ok, but the important factor is that the heat effects the PSI. You have whatever heat in a racing situation and therefore adjust your PSI to that heat level.

In a none racing situation, it's easy for the manufacter to say, set the PSI at this level when the wheels are cold. And thats it: simple.

 

When mine goes to the dealer, I always have to remind them to leave the tire pressure at 32 psi, since they will automatically inflate them to 37 as per the door sticker. And they will fill them to 37 psi cold.

On our rough roads, 32 psi (cold temp) has worked well for me for almost 30k miles and the tires have worn evenly.

 

Well at least they're trying to give you good service, most would do the check in the box like they have checked or topped something off when in fact they did not.

 

Dealerships always underinflate truck and SUV tires so the ride feels better when you test drive it.