Fascinating stuff and having been round this bouy a few times myself, I'll add my experiences.
Firstly picking up on Colin's (S-J's) post, over the years I've accumulated a number of pressure gauges. I've two of the old stick gauges with the extending scale (which are roundly criticised as being inaccurate), two newer supposedly high accuracy digital gauges, two gauges on compressor based tyre inflators, and finally a Race Tech analogue gauge from my racings days.
First thing to say is that the old stick gauges are calibrated in 1psi or 0.1bar increments, though you can "read" to 0.5psi or less. They both routinely show the same pressure and more over they offer repeatable results (i.e. the same pressure on multiple checks on the same tyre) though what they measure in absolute terms I'll come back to.
For the newer digital gauges, one measures in 0.5psi and the other 0.1bar increments. However, despite them both claiming high accuracy and zero calibration at each use, they are both incapable of giving repeated results at the 0.5psi level, often varying by as much as 1or 2psi. All very frustrating, and completely undermining confidence for me personally.
The Race Tech gauge is very clear, and gives very accurate repeatable results, and you can "read" the pressure to 0.25psi, but it was bought for racing slicks and wets operated at 12 to 15psi depending on conditions and has a full scale selection (FSD) of 40psi. Best practice suggests that mechanical (i.e. Bourdon Tube) gauges are at their most accurate in the range of 40 to 60% of FSD. So, this is fine for the Plus 4 at ~22psi, but for the BMW tin top running 2.2 or 2.6bar (front) and 2.3 or 3.0 bar (rear) depending on load, it's outside it's operable range.
Going back to the absolutes, surprisingly the two old stick gauges and the two compressor tyre inflators all read the same pressure for a given pressure setting on a tyre in the normal 25 to 35psi range. Only as the pressures are increased to 45psi (3bar) or over is there any marked separation in the readings, and then only by say 0.1bar or 1.5psi. However, when compared to the Race Tech gauge at the 20 to 25psi range they all tend to read about 1.0 to 1.5psi low. So which is right in absolute terms? Well in truth I don't know, so I use the Race Tech gauge for the Plus 4 and the stick gauges for the BMW... I've given up on the digital gauges because of the poor repeatability issues.
In the Mog, for 99.9% of the time we are two up in the car and I set them at 22psi all round. I've tried all combinations of 20 to 26psi settings all round at 1psi increments and with up to a +4psi rear bias over a period of time. For my taste >22psi is too hard, bouncy and uncomfortable, <22 psi is too soft with too much sidewall deflection. I also check my tyre temperatures (admittedly only by hand rather than an infrared thermometer) and find that at 22psi they are, as far as I can detect, evenly warm across the tread front and rear. In absolute terms, assuming the Race tech gauge is the most accurate, you could say that this equates to 23 to 23.5psi on the stick gauges.
Now, who was is that said it's a black art...
P.S. My post was in response to S-J... Slow typing!
