I have a tire on my pickup that has a slow leak, I've looked several times but can't find any nail. The first time I noticed it was after towing an empty trailer, the truck was all over the place from the imbalance over bumps wagging the trailer which wagged the truck. However, it didn't look flat, just slightly bulged, I checked it and sure enough, it was 10-15psi, I can't remember.

Anyway, I filled it and things were fine. Months later (today) I happened to look at it and it looked low so I checked it and sure enough, 15psi. I filled it up again, but that got me thinking. Googlefu says sea level psi is 14.7psi, we are roughly 600ft elevation, so not much change from that. When I checked it with the tire gauge, there was a "hiss" when I removed the tire gauge, so there was more pressure in the tire than outside. I assume the additional side wall construction of a pickup tire might actually hold the weight of the truck and not go truly flat.

My question is, and this is probably junior high science, but its been awhile...that tire still had 15psi GREATER air pressure than the atmosphere outside the tire, correct? I mean, assuming you have a tire that is rigid enough to support the weight of a vehicle regardless of air, if there is 14.7 psi pushing in on the tire from outside, and you have 15 psi pushing out from inside the tire, would the gauge still read 15 psi or would it read 0.3 psi?

Does a tire inflated to 36 psi have a 36 psi pressure increase from outside the tire or only 21.3 psi? I've had flat tires, balls, etc read 0.0 psi, but the closeness to atmospheric pressure made me wonder.

There’s absolute pressure (pressure compared to vacuum) and gauge pressure (pressure compared to ambient pressure).

Gauge pressure is called gauge pressure because almost all gauges measure gauge pressure.

A tire gauge measures gauge pressure, a reading of 15 PSI means 15 PSI higher than the atmosphere outside the gauge.

I have no clue on the scientific portion,

But regarding the leak, it could be the tire stem there was a batch of defective ones a while back, or leaking around the rim somewhere.

Bottle of dish soap and water, fill the tire, then spray the tire around the rim, inside and out, and the stem. If if you can find a leak.

My 20 inch rims have a tendency to leak slowly if I let it sit a while around the rim, a common problem with those chrome rims

Related to your issue, not the question.

This may not / probably not be your issue. However, one might check the bead area on the tire that seals against the rim.

Seems like there is a barcode sticker there that should be flat against the tire. But it seems that sometimes this is raised above the tire surface and may result in slow, intermittent leaks. Check for this if you remove the tire from the rim.

PSIg & PSIa - gauge & absolute.
You read PSIg with a tire gauge, which ignores atmospheric pressure.

Pssssssssss is the key?

If you press the valve and air comes out, inside the tire is higher pressure than ambient air (tire PSIg > 1ATM). If no sound, then the pressure inside is the same as ambient and stasis is reached - doesn't mean there is no air in the tire and no pressure in the tire, just that it's equal to ambient (PSIg = 0; PSIa = 1ATM). If the tire sucks in air, then the inside is lower pressure than ambient; ambient is forcing its way into the tire (tire PSIg < 1ATM).

The tire doesn’t carry the weight. The tire holds the air which carries the weight. That’s how pneumatic tires work from bicycles to the biggest mining dump trucks.

Think of a balloon filled with air. You can rest the balloon on a table and rest your hand on the balloon. Pop the balloon and you hand will hit the table. The balloon itself does not keep your hand from falling, rather it only holds air. It’s that air that keeps your hand from falling. Pneumatic tires are just fancy balloons.

I worked at a gas station in high school, (1977) and the owner also repaired and sold some tires. When he put 30 pounds of air in a tire he literally thought there was 30 pounds of air in the tire. I couldn't convince him otherwise.

I had a slow leak with my truck 2 years ago. I had to add air every 10-14 days. I put up with it for 6-9 months, couldn’t find anything.

At the direction of a relative, I took the tire off, to give it a close inspection. I had the soapy water ready. Low & behold there was a finishing nail buried to the stained head, hard to see.

I pulled the nail, plugged it myself, fine since.

Thanks everyone for confrming my science question. On the air vs the tire holding the car up, absent a run flat tire, wouldn't a tire with enough layers of sidewall do that to some extent, even reading 0.0 psi? That's what I was also wondering. I've had flat tires before, but it's usually within hiurs/ a day, flat as can be 0.0 psi, car sitting on the rim. I've never encountered this situation on this truck before.

Way before my time, tires had "ply" ratings that were literally how many layers the tires carcass was made of. The more plies a tire of a given size had, the higher the air pressure it could hold, the more weight it could carry. Most tires' carcasses now only have 2 plies, they just make them stronger to hold more air, so tire manufacturers started use load range instead of ply ratings.

It goes a little like this:
Ply Rating Load Range Max PSI
6 ply C 50
8 ply D 65
10 ply E 80
12 ply F 95
14 ply G 110

My 5th wheel came with Load Range E tires made with 2 plies I replaced them with Load Range G tires of the same size also made with 2 plies. The difference being the E tires where made with 2 plies of polyester cord while the G tires were made with 2 plies of steel cords. The G tires sidewalls are much stiffer and the tires weighed 60 pounds each vs the 35 pounds of the E tires.

Now here's the interesting part: the amount of weight both tires are rated to carry if both tires are filled to 80PSI is exactly the same. Why? Because the air carries the load.

That's a long winded explanation for: if both tires were flat, 0PSI, then the one with the stiffer/stronger sidewall would look less flat, but only because it's stiffer/stronger to hold more air, not to carry more weight. Both will be destroyed if you drive on them with no air.

"Bar" walks into the conversation. Looks around and walks out.

quote:

Originally posted by myrottiety:
"Bar" walks into the conversation. Looks around and walks out.

Must have gotten psiched out.

Remember you are supposed to check your tire pressure when cold before driving. As driving even just a few miles especially during the summer can raise your tire pressure a couple of PSI. God Bless