If you have/take a half-dozen gauges and test them against each other how do you know WHICH if ANY of them are truly accurate? You don’t. Without a recent standardized calibration they’re only representative. I am not so much interested in accuracy (within reason) but precision.
The TST TPMS on our old 1575 consistently showed 5 to 7 PSI lower with the three gauges I had. However, the gauges and TPMS were always consistent.
The on-board TPMS on my new dually totally sucks, however, it sucks consistently. After filling the six tires to 80 psi cold (using one gauge) the on-board TPMS will show the six tires varying from 73 to 78 psi. After the tires warm up in a few tens of miles the TPMS will stabilize at, say, 88 psi for all six (weird I know). After driving the rig for 8K miles, like Randy said above, I’m comfortable with what I know about my tire pressusres.
Dave