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Jim Bettison
Posted on Friday, 10 August, 2001 - 21:31:   Edit PostDelete PostView Post/Check IP

The other day when I was trying various tyre pressure combinations for our MkVI I found that two tyre gauges were giving me substantially different readings.
So I collected all the gauges we have - all reputable makes, including the gauge from the MkVI tool kit, and the gauge of the tyre inflator run from a compressor - about 5 in all. I found that a pressure in the tyre that I thought was 32psi was indicating 28, 30, 30, 32, and 34 - depending which gauge I chose ... that's about 18% variation!
This worries me and makes me wonder how many people are experiencing what they believe are tyre problems but are really gauge problems. And I know that many current tyres can be very tetchy about the air they contain. To compound the problem, I rang the local road service/auto association (Royal Automobile Assn of SA) whe told me that a service they used to offer had been terminated ... I'd be very welcome to bring my gauges and compare them with their workshop units (and I appreciate that offer) but the tech officer to whom I spoke didn't know when or if their gauges are calibrated or of anyone offering a calibration service.
Does anyone share my concern? And, what is to be done?
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Martin Cutler
Posted on Tuesday, 14 August, 2001 - 13:25:   Edit PostDelete PostView Post/Check IP

Interesting point Jim. (Sorry I couldn't help you on the timing question, by the way). I always check my tyres at the same garage, so at least I am using the same gauge. The gauge I have at home was my grandfathers, and is the pencil type with a slide that pops out. It seems to agree with the local BP servo. The ones at the BP are the new ones, where you type in the pressure you want, and when the desired pressure is reached, it beeps at you. Very techie!
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Bob (
Posted on Sunday, 19 January, 2003 - 02:54:   Edit PostDelete PostView Post/Check IP

I have come across this on many occasions.
You can get accurate calibrated tyre guages these are pricey. ( £100 plus)
They also have to be checked at certain intervals.

main probelm is cause by rough handling of guages by people who drop them on garage fore courts.

I have a gauge which I adjusted to show exactly 30 psi using a master guage which I borrowed from the police workshops. --- how nice of them.

So now I know that at 30 psi the guage is correct.

So I can be reasonable close at other near pressures.
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Mark Tomlinson
Unregistered guest
Posted From:
Posted on Tuesday, 15 July, 2003 - 22:27:   Edit PostDelete PostView Post/Check IP

To avoid having to check tyre pressure all the time take your car to Bob Jane T mart and have them fill the tyres with Nitrogen @ $5.00 a tyre, they do not loose presure and are much better to drive on. Nitrogen is used in aircraft tyres and racing vehicles and is becoming more commonly used in heavy passenger vehicles lately, I had the shadow done and the difference was very noticeable.
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Chris Gillings
Username: chrisg

Post Number: 14
Registered: 5-2001
Posted on Wednesday, 16 July, 2003 - 01:46:   Edit PostDelete PostView Post/Check IP

My first reaction to this was "What a great idea!" and I started to think of the possible difference in thermal behaviour of nitrogen. So I googled for "nitrogen in car tyres" and the first two unrelated links were these:



So the jury was still out. I needed more technical information. A simple change to American spelling - "tyres" to "tires" - readily googled the following link:


So all we have to do now is calculate the expansion coefficient of the volume of nitrogen in our tyres to see exactly how much variation we can expect. Formula 1 tyres would have comparable volumes to our own, but they are subjected to the most extreme conditions, which I should hope ours aren't!

I suspect it's pretty obvious that continued expansion and contraction of the air in a tyre exacerbates leakage, but as the man says, there's no substitute for maintaining your tyres and keeping an eye on the pressures regardless of what's in them.

Just a thought: if you find yourself out somewhere with a leaking tyre and can't top up with nitrogen, would you feel obliged to equal up the mix on all the tyres? You'd have to let them all down to the same level and then top them up again, which seems odd to me for some reason.

Another thought: if a contributing factor is the presence of water vapour in ordinary air (see the third link), surely it's easy to build a compressor that desiccates its output? Wouldn't compression naturally desiccate air?

Anyway, I can't say I'm in a hurry to get nitrogen, but I hardly think light truck crossplys qualify anyway.
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Mark Tomlinson
Unregistered guest
Posted From:
Posted on Wednesday, 16 July, 2003 - 09:13:   Edit PostDelete PostView Post/Check IP

I took the option on Nitrogen simply as an experiment. Should I find a leaking tyre I would change to the spare which has also been filled with Nitrogen till the leak was fixed. The "advice" given to me by Bob Jane T mart staff seemed honest remembering I aproached them so $25.00 seemed cheap and the noticeable difference was that the car seemed to ride just a little more firmly on the road making the car feel safer.
Given that I have only owned the car a month I am still very wet behind the ears and am only making the comparison to my other heavy car which is an 89 fairlane, a lightweight in comparison.