ecohen2

I am currently on a long trip and noticed that the built in tire pressure gauges all read about 4 psi low compared to my handheld gauge. Is this normal?


Of course at 3am I am going through all kinds of ways to figure out which is right. * If the TPS was broken then only one sensor would be off by 4 PSI.
* If the gauge was broken, it would read high for all of them, but I checked the gauge against another and they matched.
* I am not running the stock rims, so maybe the aftermarket sensors are all wrong.

and so on and so on.. I guess ill just buy another gauge today... Has anyone had this happen?

Ed

I am Jeff

My dash reads a few pounds lower than what I get with my tire gauge. I don't recall the exact amount that it's off by.

Mort

Are you measuring cold or hot pressures? Does the TPMS reading go up as the air in the tires heats up? Maybe you are comparing hot tire air pressures with cold tire pressures as measured by the TPMS at cold start up.

jmueller

TPMS pressures change while driving.

jvanbrecht

Air pressure within the tires changes, not TPMS (sorry, I know, knit picking)..

Regular air fluctuates with temp, while in motion the temps rise due to friction, which will result in a higher pressure being monitored by the TPMS, when you stop, it can change somewhat quickly by a small value, which would explain your 4 psi discrepancy.

If you want more stable pressures, use nitrogen. No it won't make you go faster, no it is not lighter than air (which is 70% nitrogen anyways). Nitrogen does not compress, and is stable over a much greater temperature range so it will not expand or contract, which is what the oxygen portion of air does. It also has larger molecules, so less likely to have oxygen leaking through the tires (very very small amount, but it does occur, and we are talking on a molecular scale here). This is why in Winter your TPMS warning goes nuts in the morning, but after reaching driving speeds, the pressure goes back up beyond the warning threshold (although you are still going to need to put air into the tires)

jmueller

To be clear.. The readings on the dash within the monitoring system do actively monitor the fluctuating pressure inside the tires and will reflect those changing pressures on the fly. This makes sense as when you're driving and get a flat tire the system notifies you of the fact that you should not attempt a burnout.

ecohen2

Here is an example.... This morning I checked my tires with the gauge before I even started the car. The car had been in the garage all night long and it was about 70 degrees. I had 45 PSI all the way around. I drove the car for about 10 minutes and looked at the onboard system and it showed 41... So it is at least 4 degrees lower since the tires had actually heated up by the time I checked..

I am actually considering Nitrogen for a bunch of reasons, mostly to reduce leaking and changes due to temperature...

Thanks for the thoughts....

Ed

bhamg

FWIW I had my nitrogen-filled tires on another car topped off at 36psi after a 6 mile drive to the filling station, ambient temp of 86º. After sitting for two days I checked them in the morning, ambient 71º in the garage. all four were 33psi. That was a spread roughly what I've seen with just regular air-filled tires, so rather disappointing. I judge nitrogen stability to be a bit of a myth.

ecohen2

Who did your fill? I know if you don't get the moisture out of the tires before you do the nitrogen fill that it does not work as well.

Tirerack has a nice piece on their site about the physics etc about nitrogen...

D C

For a road car, it's not worth the hype IMO. On race cars is where it's most noticeable, and it does work well. Yes there is still change from heat, but the swing isn't nearly the same as with air. But your also looking at a system where the tire is purged and filled with N2 from the get go. I don't know if places that fill passenger car tires actually purge the air out.

bhamg

This was done at Costco - PSS on a set of brand new wheels that I brought to them - mounted on a typically sunny and (too) dry SoCal day. BTW for those of you who worry about installations there I asked the manager for his "best guy." It was one of the very few times I could not find even the slightest clamp mark on the rim, anywhere. So I'm a Costco convert but in general, Discount Tire has been fantastic over the years.

glennhl

My onboard system reads about 3 to 4 psi low. Funny thing, it was the same error on my 2006 Vette.

As far as Nitrogen, it acts like an ideal gas at the pressures we are talking about and the pressure will increase as a ratio of the absolute temps. So if it's 70 degrees out, that's 530 Rankine absolute. If the nitrogen inside the tire heats up to 120 degrees, that's 580 Rankine absolute. If your pressure at 70 degrees was 40 psi, then your new pressure will be (580/530) x 40 = 44 psi.

Now as far as using air? Guess what, it also acts like an ideal gas and the pressure will increase exactly the same. HOWEVER, that's for dry air. If the air has any water vapor in it, then the pressure change can vary slightly more due to the partial pressure of the water vapor.

Personally, I think using Nitrogen is a waste of money.

jcfay

FYI all, the show fifth gear (UK) did a good bit on the nitrogen tire thing. Their testing showed that most nitrogen fill stations weren't providing either pure nitrogen or were contaminated with water vapor. I think nitrogen would be great but I'd be really suspicious of actually being able to get it, and being able to have a fully empty, fully water vapor free tire filled with it. IMO, air is just fine

Ingenieur

The atm is -80% N2
The advantage is moisture
If your shop doesn't use an air dryer find one that does
Air is fine as long as it is dry
When pumped from a tank it may have a lot of condensed water

bhamg

Based on this it sounds like my nitrogen was "dry!"