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I have some experience with the OBDLink MX+ and TPMS. Here is what I can tell you.
From my readings comparing the MX+ with my tire pressure gauge, the MX+ reads about 13psi high. So if it's reading 55psi, that tire would be around 42psi. When the sensors are not reading (i.e. just turned van on), they report 14.5psi if I recall correctly.
From my experience, if I turn the van ON (engine not running) and start up the OBDLink app, the TPMS values will change from 14.5psi to an actual reading within 1-2 minutes, without starting or moving the van. At least that was the case with the sensors in my van previously (I assume they were Toyota sensors). I just changed out all 4 sensors for Autel MX-Sensor. It remains to be seen if the Autel's are any different.
I suspect it's exactly a 14.7 psi difference, and the pressure read from the MX+ is displaying psia, not psig, as 0 psia = -14.7 psig, and 0 psig = 14.7 psia. When most people say "psi" they are referring to "psig", or gauge pressure, which is the pressure difference from atmosphere to what you are reading. Psia, or absolute pressure, is offset by 14.7 psi because it's the pressure relative to a vacuum, and standard atmospheric pressure at sea level is 14.7 psia.
14.7 being the difference between TPMS reading and MX+ reading, may be slight variation between TPMS sensor and your own pressure gauge.
Yesterday I got my own MX+, and put two sensors on the app. I read 48.7 and 49.1 psi. I then used my Ateq VT31 tool to scan the sensors, and of the three working (I think the tire shop messed up one TPMS sensor) I got readings of 34.4, 34.6, and 34.0 psi.
48.7 - 14.7 - 34.0, and 49.1 - 14.7 = 34.4. This exactly lines up with two of my tire pressures, so I'm going to say this accounts for the MX+ pressure reading high. Now I just need to figure out how to subtract 14.7 from the tire pressure display in the OBDLink app.
