Tyre Pressures

[BfgSLKMan]

It's like anything if you don't keep them calibrated then how accurate are they

I am an Instrument Tech and i do calibrate gauges. Unless it is calibrated on a certified piece of equipment like a Druck Pressure Calibrator then how would you ever know.

A couple of p.s.i drift on a Bourdon Gauge is not uncommon. Generally these things have a +/- 10% tolerance

I live in germany and by law all air lines at forecourt garages are calibrated/checked every year with a sticker to show they have been done - they are usually done at the same time as the fuel pump checks (also have a stamp on them somewhere). I only ever use one garage and it reads the same as my car. Luckily I only really every need to adjust my tyre pressures when the summer/winter wheels go on, as currently all of my alloys seam hold air pressure without issue.