Tire Pressure - but wait... a different question!

I keep my load range E tires at 80 PSI rear and 65 PSI front as recommended on the door jam sticker. I always check and fill first thing in the morning while they are cold. The manufacturer understands that a load range E tire will carry a load and that they will build pressure with heat. Follow the recommendations for you set up as noted on the sticker inside the drivers door jam and don't worry about it. I have no proof but I would guess many more tires fail by being under inflated than over inflated.

I am amazed at how much some people fret over issues like this but use a gage they picked up for a couple of dollars somewhere. Most likely your gage is the largest source of error. I don't think it is even possible to get one calibrated but you should check your gage against two or three others to see if you get consistent readings. I have seen gages vary by 10PSI.

2 thoughts:

On passenger car tires, the difference in pressure between loaded and unloaded is less than a 1/2 a psi (based on actual measurements) - well within measurement tolerances. That's about 1 1/2%, and that would make sense compared to the 4% for aircraft tires, which are more intensely loaded.

ALL tire pressures are "COLD" pressures - except when talking about pressure buildup in service. The MAX pressures listed on the sidewall of tires are COLD pressures and the tire manufacturer factors in the amount of pressure buildup when those are specified (Actually, it's the reverse of that - the tire is designed to withstand those elevated pressures.)

Campin LI wrote:
When setting the tire pressure, do you set it when the truck is loaded or when it is empty

Either works fine as they both will be the same. I can load my 8000 lb tractor on my GN trailer and tire pressure cold set at 80 psi is still 80 psi even with the tractor sitting on the rear of the trailers deck and mashing the tires on the rear axle nearly flat. No I don't pull with it sitting there.

I'm assuming empty but a co-worker says the pressure in the tires will be higher after it is loaded (if you pump tires to max psi, they will be over max psi and then when they heat up they will be way over). He is probably right but is it a negligible difference, anyone know?

Tires will heat up with highway speed/sun which raises pressures. All is normal as the tire are designed to do this. If you notice tire makers say to cold set your tires pressure and not bother them.
I've seen my E tires on my truck and equipment trailers go to 90-92 psi at highway speeds on a hot day at max axle loads. The next morning their back to 80 psi.

Both sides of the argument are at least partially true. Being an old retired A&P aircraft mechanic, I remember a quote that says you add 4% to the pressure if the specification was for unloaded pressure and the tire is actually loaded. I know some will hurl stones because we are talking about auto tires, not aircraft tires, but the principle is the same. However, aircraft tires are designed with about twice the deflection as auto tires, so the effect is much less for auto tires.

So, YES, the pressure does go up a tiny bit. Not enough to worry about with the "crude" gages most of us have.

Here's a link to the article and a quote from it:

"4. INCREASE PRESSURE 4% FOR TIRES UNDER LOAD
It must be determined if “loaded” or “unloaded” pressure has been specified by the aircraft manufacturer. When a tire is under load, the gas chamber volume is reduced due to tire deflection. Therefore, if unloaded pressure has been specified, that number should be increased by four percent (4%) to obtain the equivalent loaded inflation pressure. The opposite is true as well: if loaded pressure has been specified, that number should be reduced by four percent (4%) if the tire is being inflated while unloaded."

Now you know your co-worker isn't all that bright........

The Internet is an amazing thing:

Tire Pressure vs Weight as told by Bridgestone.

For good measure GY has some good info:

GY RV stuff

For tires, my rule of thumb has been to fill them with air until they don't get hot when driving. The contact patch of the tire is the only place where you want the tire to flex, but not too much. I adjusted my tire pressure until I could drive it 60 miles and it wouldn't be hot to the touch.

40K miles later my tires are wearing evenly, granted I get my tires rotated every oil change.

There are three primary variables related to tire pressure. Pressure, Volume and Temperature. If you'd like to know the science behind it, check out this article:
Ideal Gas Law

azdryheat wrote:
Airbags are not the same as a tire. The entire airbag is compressed from weight above while a tire is only compressed on the lower portion of the tire. That's why a tire only goes flat on the bottom, lol.

Correct answer here! Check your tires when empty and then recheck them with the truck fully loaded (assuming you haven't driven on them and ambient temp is the same) and the pressure will be exactly the same.

The airbag is getting squeezed as the suspension is loaded, and less volume = more pressure. So not surprised 10 psi empty becomes 20 psi loaded. The size of the tire is not changing, therefore it's pressure can not change unless the temp changes.

Go tell your co-woker ha-ha nice try but you ain't falling that old trick. Then ask him if he has two 10's for a five.....