Amongst my tools, I have a beautiful four inch pressure gauge which reads from -1 bar to +5 bar in widely spaced 100 millibar ( 100 hectopascals to be up to date) graduations. It is calibrated to national standards and the calibration is current. The individual TPMS sensors can be up to 200 millibars away from the gauge reading, which is about 3 psi in old money so if you are setting up your car for roadholding with bushes, dampers and the like, I would strongly advise you to buy a good pressure gauge!

Being old school (or just old I suppose)  I still work in psi.  I've always used a good quality pressure gauge as I don't trust the ones on garage forecourts, they get thrown about a lot.

I did have a gauge many years ago which I had checked against some laboratory pressure measuring equipment which was NAMAS accredited (think they are known as UKAS these days)

My current car is a Mazda 3 which according to the door sticker should be 38psi front, 36psi rear.  If I set it to that with my gauge, my dash readout says 44psi front 40 rear.

Whenever I take it for a service the garage always write on the service notes, "reset tyre pressure to manufacturers reccomendations on door sticker" , which they claim is 36psi all round.

Not sure what door sticker they are reading but it's certainly not mine. 

When I then check them with my gauge after the service they are usually showing 32psi all round so I don't know if they have a bad gauge or they are using the dash readout as when they set them to 32 my dash says 36.

 

 

It appears that your gauge tallies because it reads four pounds low when the TPMS is set to 36 and when 36 is set to your gauge the TPMS reads 4 to 6 pounds high. I find the Fiesta to be moderately sensitive to tyre pressure and set to the gauge figures, itseems to work better although I find that my 2.1 Bar often comes up at 2.4 through the TPMS.

Manufacturer's recommendations may not be the best.  For an oldie like me, the Autocar roadtest of a 2.8i Capri (I had two of them!) suggested that 24psi was better than the Ford recommendation of 28psi.  With 28psi the centre of the tread would wear out very quickly, lasting only about 12k.  With 24psi tyres lasted about 20k.  Ride was better, with no noticeable change in handling. I would think that manufacturers may have recommended a higher pressure than optimum, because it is only anoraks that check tyre pressures frequently with a quality gauge, so most car tyres ended up under inflated!  Why TPMS should over read I have no idea.  There is a lovely term, "Mesurement Uncertainty" which very few people appear to have heard of......

I haven't read an Autocar road test in years, I don't know if they test cars as hard as they used to!

Minus 1 bar - so vacuum gauge capable ?