Tire Pressure Lights and Tire Pressure

[jonl9]

For the second time the tire pressure indicator came on. The first time it was one tire that was low. The dealer checked the tire, all was ok, inflated it and I ran fine for several months.

Concurrent with the first real cold morning, the light came on again. Two pressures were low, one more than the other by several psi and was what I remember as the tire that was low the first time. They all were a bit low so I inflated the tires to 32 psi. After a couple of miles, the light went out as it should have. But, I brought the car in again for a tire check, which they did. No problems found.

All tires were lower in pressure than 32 pounds (sounds like a guage problem) and the suspect tire was lower than the rest (this could be a DUP (Dumb User Problem)). The dealer inflated the tires to 34 to 35 psi all around and I'm going to watch them for a while to make sure there's isn't a real problem.

I have had change of season problems with other cars in the past with the indicator light and yesterday morning the dealer had a line of them. Their recommendation for that was nitrogen in all the tires. I don't intend to do this, if there is truly no problem with the tires they should be checked and inflated periodically anyhow.

Now, something I noticed. It could be akin to the sense that a freshly washed and shined car feels like it runs faster. But with the increased tire pressure, the tire tread noise seems significantly less and a low road vibration that I just accepted as normal ride for this car is gone. The car seems better at highway speeds with the slightly elevated pressure done by the dealer.

Hmmmm, seems the car is fairly senstitive to tire pressure.

Jon

[jimkberry]

First: I have an '11 SES Hatch with 16"s (well, it _came_ with 16's...) and so that's all I really know about for sure but:

a) Check the sticker inside the driver door frame to see what your correct tire inflation pressure is: I'll bet it's 35lbs. If you only inflate to 32lbs you're already 10% low. It's a shame the dealer didn't know any better. The door frame sticker is the definitive source for recommended pressure on any car.

b) When I bought TPMS sensors for my new wheels I noticed that part of the item description was "35lbs", which I take to mean that 35 is the sensors' baseline pressure. once again: by starting at 32 you are already 10% low - I'm not sure what the threshold for a warning is, but I beleive somewhere I have heard 20% (yeah, that's some good fact-finding, right?)

c) If you don't have one, buy a cheap digital pressure gauge next time you're in a department store. Even a cheap one is way better than the "pen"-type mechanical ones, and ANY gauge is 1000X better than none.

d) You will _definitely_ notice a difference in ride and noise - and fuel economy, too - going from 32 to 35 lbs. I typically run 36 in my tires.

e) There is no "e". I wonder why I decided to use bullet points like this for this post?

-jim

[Iceracer]

Tires lose 1 psi per every 10 degree drop in pressure.

I found that running my tires at 36 psi works very well.

The only time my light came on was when I had picked up a sheet metal screw in the center of the tread. I feel that it saved me a tire or at least having to change it on the road,

[Iceracer]

Originally Posted by jimkberry

First: I have an '11 SES Hatch with 16"s (well, it _came_ with 16's...) and so that's all I really know about for sure but:

a) Check the sticker inside the driver door frame to see what your correct tire inflation pressure is: I'll bet it's 35lbs. If you only inflate to 32lbs you're already 10% low. It's a shame the dealer didn't know any better. The door frame sticker is the definitive source for recommended pressure on any car.

b) When I bought TPMS sensors for my new wheels I noticed that part of the item description was "35lbs", which I take to mean that 35 is the sensors' baseline pressure. once again: by starting at 32 you are already 10% low - I'm not sure what the threshold for a warning is, but I beleive somewhere I have heard 20% (yeah, that's some good fact-finding, right?)

c) If you don't have one, buy a cheap digital pressure gauge next time you're in a department store. Even a cheap one is way better than the "pen"-type mechanical ones, and ANY gauge is 1000X better than none.

d) You will _definitely_ notice a difference in ride and noise - and fuel economy, too - going from 32 to 35 lbs. I typically run 36 in my tires.

e) There is no "e". I wonder why I decided to use bullet points like this for this post?

-jim

The listed pressure for the 15 inch tires as on the SE is 32 psi.

[jimkberry]

Oh, and another thing that just occurred to me:

The pressure of an ideal gas is directly proportional to temperature (measured in kelvin.)

Now, I don't know where the air comes from when a dealership fills tires, but if it's from indoors it is probably at more or less 68 degrees Fahrenheit (293k)

If a tire filled with that air (or nitrogen, or whatever) at that temperature is taken outside in the winter an let sit where the ambient temp is 32F (273k) you can expect the pressure to drop by 1-(273/293) or about 7%.

Just a thought.

-jim

[jimkberry]

Originally Posted by Iceracer

The listed pressure for the 15 inch tires as on the SE is 32 psi.

Well, "poof" goes that idea. My 16's came from the dealer at 32.

Do the 15's use a different TPMS sensor than the 16's?

...and now someone is going to inform me that compressed air in a shop always comes from outside, too...

-jim

[Norm201]

Originally Posted by jimkberry

Well, "poof" goes that idea. My 16's came from the dealer at 32.

Do the 15's use a different TPMS sensor than the 16's?

...and now someone is going to inform me that compressed air in a shop always comes from outside, too...

-jim

Well only if you don't pay for the premium stuff.

[FORDSVTPARTS]

Originally Posted by jimkberry

Oh, and another thing that just occurred to me:

The pressure of an ideal gas is directly proportional to temperature (measured in kelvin.)

Now, I don't know where the air comes from when a dealership fills tires, but if it's from indoors it is probably at more or less 68 degrees Fahrenheit (293k)

If a tire filled with that air (or nitrogen, or whatever) at that temperature is taken outside in the winter an let sit where the ambient temp is 32F (273k) you can expect the pressure to drop by 1-(273/293) or about 7%.

Just a thought.

-jim

Except that the ambient air then gets run through a compressor which greatly increases it's temperature as it's compressed and forced in to the storage tank.

On the other hand as the pressure is released the air will come out of the air hose at substantially less than even the ambient temp in the storage tank.

Best bet is to just periodicaly check your tire pressure to see what it's at, of course if people could actually manage that the feds wouldn't have mandated the sensors in the first place.



.

[FORDSVTPARTS]

Originally Posted by jimkberry

...and now someone is going to inform me that compressed air in a shop always comes from outside, too...

-jim

See my last post

[iamz]

Originally Posted by jimkberry

Oh, and another thing that just occurred to me:

The pressure of an ideal gas is directly proportional to temperature (measured in kelvin.)

Now, I don't know where the air comes from when a dealership fills tires, but if it's from indoors it is probably at more or less 68 degrees Fahrenheit (293k)

If a tire filled with that air (or nitrogen, or whatever) at that temperature is taken outside in the winter an let sit where the ambient temp is 32F (273k) you can expect the pressure to drop by 1-(273/293) or about 7%.

Just a thought.

-jim

Except you're not exchanging out the air, you're only adding air. So if the pressure was at 28 psi and you raise it to 32 psi, you've only added roughly 15% more air, not 100%. So the higher 68 degree temp of the added air would have a minimal effect on the overall psi of the tire when it cools back down to ambient.