Car Tire Pressure - how high do you go?

[leehljp]

Two things:

I noticed on our Honda back in the 100° days that the tire pressure at 36 PSI rose about 2° on average after about 100 miles.

I had to replace the tires on my pickup 2 years ago because they were about 10 years old but still had half the tread. Don't trust OLD tires!

[LCHIEN]

For normal use, my practice is to inflate the tires on my vehicles ... to 2psi more than is shown on the door jamb. ...

I over-inflate a little because it helps mileage, it helps make up for the fact I don't check it as often as I should and it goes under some too, and it still rides OK.

...
Jim

I did say previously I follow the car manufacturer's suggestions... which was more or less true.
But I confess, this is exactly what I do, too. Same reasons.

[cabinetman]

I had to replace the tires on my pickup 2 years ago because they were about 10 years old but still had half the tread. Don't trust OLD tires!

Old tires could be unsafe. They may display some checking and/or cracking on the sidewall. If you buy a used car, or have old tires, it's wise to check out the tires even though they may show minimal tread wear.

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[LCHIEN]

Two things:

I noticed on our Honda back in the 100° days that the tire pressure at 36 PSI rose about 2° on average after about 100 miles.

I had to replace the tires on my pickup 2 years ago because they were about 10 years old but still had half the tread. Don't trust OLD tires!

Your numbers seem low. Pressures in an enclosed volume rise when the gas inside is heated - a fundamental law of physics of gasses. This means a heated tire will have more pressure than when it was filled at a lower temperature.

In texas we get days where its 110F air temperature.
Then the blacktop roads get to 140F surface temperture, in contact with your tire.
If your tire is flexing continuously and scrubbing on the road because its footprint and sidewalls are flexing at high speeds then it will also rise maybe another 10-20F. This is why they give you hot and cold inflation values or just tell you to check inflation pressures when cold.
Note that underinflated tires will flex a lot - this is what caused the Ford/Firestone problems - ford recommended too low a pressure for various reasons, the tires overheated more than most, and the tires were substandard for withstanding heat and flex damage causing failure.
It's not inconceivable that a tire on a hot wide-open texas highway (which was in fact a high precentage of the Ford explorer tire blowout fatalities) that the tire temperature will reach 150F to 160F in my scenario above.

I computed the temperature rise in a tire using Charles' Law:
One case where the tire was filled at 50F and 32 PSI and the other for being filled at 72F and initially at 36 PSI.



If you look at them there can be as much as 6-7 PSI rise over the initial fill pressure, when driven fast on a hot day on a blacktop highway when the tire gets to 160F.

Note the pressure rise will decrease with initial fill pressure, and decrease with higher temperature at fill. I mention the fill temperature effect becuase its not improbable that you could fill your tires at 50F on a cool evening and a few months later be on a very hot day. Simply going from 50F to 72F on a tire initially at 32 PSI will raise the pressure by about 1.5 PSI.


Also you mention old tires... it widely suggested that tires over 5 years old be retired no matter what the tread wear because the rubber gets old and develops microcracks and loses strength and other properties. It was highly publicized a year or two ago about how tires purchased at a dealer could be 2-3 years old already (old stock), meaning they only have a theoretical 2 years life remaining when purchased. I think there are date codes on the tire if you think to look at them and know how to interpret.

[cabinetman]

The tire date code.

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[LCHIEN]

thanks for the link, CabMan

[cabinetman]

thanks for the link, CabMan

You're welcome.

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[cabinetman]

Here's some interesting tire info

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[cabinetman]

I autocross the convertible occasionally and I set the tire pressure for that purpose based upon testing. There are little triangles molded into the tire at the junction of the tread and sidewall. I put the white marking stuff you use to write a price on a windshield on the side of the tire and made a run. I went up in pressure until the white stayed on the sidewall and only wore off on the tread. That was at 40psi. I take it back down before driving home (34 on the fronts, 38 on the rear since the door jamb says 32/36).

I autocross too, and that marking method is a good indicator of tire pressure evaluation. It may though only be an indicator for the transitions that have the greatest G force on the course. For the rest of the course, a happy medium seems to work best. For racing or track events , I take across the tire tread temps... on the outer edge, the middle and the inside, to get a temperature reading per tire. The optimum would be if the three measurements are very close, which would mean that the tire pressure is good.

But, all 4 tires have to be tested as soon as I pull off the track. It's helpful to have a crew member right there with a tire temp gauge, as by the time I release the belts and climb out, the temps can change.

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[leehljp]

Your numbers seem low. Pressures in an enclosed volume rise when the gas inside is heated - a fundamental law of physics of gasses. This means a heated tire will have more pressure than when it was filled at a lower temperature.

In texas we get days where its 110F air temperature.
Then the blacktop roads get to 140F surface temperture, in contact with your tire.
If your tire is flexing continuously and scrubbing on the road because its footprint and sidewalls are flexing at high speeds and cold inflation values or just tell you to check inflation pressures when cold.

I am not going to argue with you on this as I don't known what the tire pressure should go up to. It is possible that the pressure sensors might not be accurate. On the other hand when going down the road under constant speeds of 65 to 70 MPH, the constant flow of ambient air at those speeds keeps the temp down.

I realized this a few weeks ago when I moved some things in my utility trailer for my daughter over 300 miles. I had just restored the bed on the 10 year old HF 1720# capacity trailer. The temp reached 105° that day. Every time I stopped, (about every 100 miles as well as when I arrived, I felt the tires on the 5 lug 12" wheels to see how hot they were as well as the wheel hub (for bearings). The tires were not much more than the ambient temp or the concrete where I was stopped. They certainly were not in the 140° or so range. I had a load of about 800 to 1000 lbs. I would have thought that the tires would be hotter, but the airplane effect of ambient temp wind did a lot for cooling.

[LCHIEN]

hank you make a good point there, the cooling from the airflow probably helps keep it cooler. Still if the air temp was 105 and the road temp 105 then the tire would be 105, too.

if you look at my chart which is solely based on the temperature of the tire and its air, then you're looking at 2 PSI rise from 72 to 105. So that says what your are seeing is right. I think the hot cold temps listed on the door jambs differs by 2-3 PSI so that more or less agrees then. Setting the air at a colder temp than 72, you could have a larger rise at 105, of course which was one of my points.

[cabinetman]

The chart to me looks like a static test. Other factors (not based solely on the temp of the tire and its air), affect the inflation/deflation figures. For example, tire construction, type of materials, load, speed (friction/resistance). In a static test, it's entirely possible for two like sized tires from different manufacturers could respond differently to the same temperature variations.

In maintaining a good regimen for tire pressures, is checking regularly with your own gauge, and to use the same gauge every time. I like an analog dial gauge versus the pencil type. The digital ones I've used don't seem to be that accurate.

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[leehljp]

hank you make a good point there, the cooling from the airflow probably helps keep it cooler. Still if the air temp was 105 and the road temp 105 then the tire would be 105, too.

if you look at my chart which is solely based on the temperature of the tire and its air, then you're looking at 2 PSI rise from 72 to 105. So that says what your are seeing is right. I think the hot cold temps listed on the door jambs differs by 2-3 PSI so that more or less agrees then. Setting the air at a colder temp than 72, you could have a larger rise at 105, of course which was one of my points.

One of the reasons that I was watching that so close is that I didn't do one of my cardinal rules of tire changing - change out old (10 years old) tires to new ones on that trailer. Those were 10 year old trailer tires and the trailer had not been used for the past 6 years at all. When I took the trailer out of storage, I had checked the bearings, repacked them, and filled the cavity with new grease. I also put a new hitch latch on it. But I couldn't bring myself to buy new tires when the original were not worn the first bit. I did use the trailer for a couple of 75 mile round trips prior to that 300+ mile trip from N. MS to Springfield MO. Being refurbished, I actually checked them at around the 75 mile, 150 and again at about the 200 miles and then lastly at the destination. I put 50 PSI (cold as in about 90°) in those tires that were rated at 80 PSI max. I forgot what the pressure was when I checked them along the way but they were close enough to that to the point that I wasn't worried about it.

[All Thumbs]

What are you guys using for gauges?

I've seen wide fluctuations from one gauge to another.

Makes me wonder which one to trust.