How accurate are tyre air pressure gauages?

[ihatecars]

My Xantia needs 33psi, had new tyres fitted and when I got home to check with "pencil type" gauage and it showed 25psi. I did a check as the steering did feel a little unusual.

So I've been back to the tyre place who were very helpful they did work on the tyres and upon return I whipped out my pencil tyre pressure gauage in front of the manager, I asked "what are these tyres inflated to?" he say 33 psi, I did the test and was 38psi
So he went into his shop and brought out a new tyre gauage (£10) did the same test, it read 38psi So he went and got his "calibrated" gauge (£100 unit used for DoT) and did a test, that showed 33psi.

I don't know what to believe anymore.

[Hell Razor5543]

Part of the problem is temperature, both ambient and tyre. If you inflate the tyres on a cold day, all things being equal (and no leaks) the same tyres would have a higher pressure on a hot day. Equally, if you inflate them when the tyres are cold, and then go for a hard motorway drive, the tyres will warm up and the pressure will increase. I try to do the tyres at the same machine at the same garage before I do a long drive.

[Northern_Mike]

The machines are all crap. They're not very accurate, they're not well maintained and don't get the air tanks and condensation drained out of them as often as they should.
Pencil type gauges are not great, and footpump gauges are woeful. There's a good range of digital gauges that start at £7 or so. There was a test in Practical Classics some time ago. They were all pretty accurate but the cheapo one made Ring was the best. I have one of these. I'll paid about £6 off ebay for it new about 3 years ago. I usually use a footpump and this gauge..

I'm just a grumpy Northern git.

[ihatecars]

hello fellow grumpy northern git Yeh thanks for the comments, although I do understand the tyre temp thing, when to take the measurements, petrol station gauges being dodgy and high/low tyre pressue not good which is why I want to get a good reading.

As for could my pencil type by suspect, the digital one did confirm it!?

[harryp]

Guys, just doing a troll and saw this post.
Useful to bear in mind that it's not uncommon for tyres to lose a few pounds after fitting as they "settle" on the rims - so do a few miles immediately after fitting, then re-pressurise. I was always told to check pressures first thing in the morning and never after a run etc. Yes they do reduce in the winter and increase in the summer due to ambient temp changes; and a pound over the recommended caters for the pound under to average out. I personally put 1 bar over pressure in when they require topping up. Same for prevention of aquaplaning .
I used to work for a major tyre company and tyre pressures are not as scientific as you might suppose

[Northern_Mike]

1 bar over pressure? REALLY?

Get a decent gauge and use it. It's not hard.

Sent from my SM-N9005 using Tapatalk

[DickieG]

As I have a tyre pressure gauge calibrator at my disposal I've tested quite a number of gauges and TBH most are pretty accurate as in within 2 lbs, however when it comes to the inline gauge fitted to tyre inflators it can be a very different matter.

Firstly having inflated the tyre in order to get an accurate reading you must briefly "flick' the inflator lever otherwise the reading will be miles out and secondly as they operate by way of bi-metallic strips, when the ambient temperature drops much below about 5º C they become very inaccurate indeed so be wary of them in the winter. The other factor with the gauges in these inflators is that they require periodic oiling,,,,

[harryp]

Sorry, sorry
That should have read 0.1 bar of course