Most, if not all, air gauges are all over the map with supposed incorrect PSI readings as you stated...I also have a few and they all vary by a couple pounds each...I have been using a digital unit from Accutire tire that seems to work the best...Only because it seems to be the one that feels the best to me...Consumer reports a while back did some type of testing with various air gauges and found that the pencil units from NAPA were the most accurate and consistent..I believe the one I have cost about $8.00 and is the best of the bunch.....Kind of a "crap shoot" in most cases I think.....I like most others check my bike/car tires on a regular basis and so far have gotten decent wear on most....Regards and no real answers on the tire gauge issue....