Tyre inflator & power supply

For a good number of years I have been nursing a £3 12V tyre inflater. It wasn't particularly good when new, but was much better than a foot pump.  I've variously replaced the piston, gauge, pipe and tyre attachment. Then the entire pump when I 'found' one without a case.
I must have blown a seal as it now takes 10 minutes to add 0.5 bar.
So I bought a new one.  There seems to be a few basic designs re-labelled.  I found a nice-on-paper Black and Decker one, seemingly a one-off.  Mains and 12V. Around €50. The reviews were awful, particularly in America. The main fault was that if it worked at all on mains on receipt, it failed pretty soon afterwards. Whilst I'm not adverse to a bit of power supply repair, it was occurring too often. The next one on the list was an AEG KD 7.0 with a digital display and auto shut-off, which was being sold for €44 on French Amazon. German amazon had 6 'good' condition ones for €12.
So I bought that. The main review complaint was noise. I also bought a rubber boot mat (who'd have thought a Audi A4 2003 boot is the same width (and twice as long) as a Captur, which worked out €15 cheaper than the Captur specific one, with the advantage I can use the extra length to protect the bumper.
The pump swallows power. At least 10A to start and 9A to run. I could have taped 2 5A/12V bricks together, but I had an old AT PC power supply. This is plated at 9A/12V. It delivered 11.2 V.  Loading the 5V rail with a 6.8R resistor got this up to 12.1V (A 4.7R got it to 12.2, a 2R to 13.5V, but the resistors were getting large and dissipating a lot of power ~13W) Snipped off all the old connectors, put on a big cigar socket.  Works well, and it's in a nice steel case.
The inflater is a tad noisy, but will add 0.5 bar in 30s. The one thing I don't like, which seems common on a lot on inflaters, is the tyre connector is screw on, rather than clip on. I could replace it, but it isn't too bad, and at least it doesn't pop off. The original cardboard carton was battered (now in a skip) but other than that it was sealed in a plastic bag and was clean. Amazon used wins again.
I do have another problem, though.  If the inflater reads 2.5b, the 'old faithful' stick gauge reads 2.6b and the new digital gauge reads 2.35b. On average, the inflater is correct....