i say a good grade hand held tire gauge.
there is such in my pick up and it is a fail.
i set the psi ( with my gauges) and the readout gives varied numbers.
then if you move the tires around. or get new tires did the tire peps put them back as how the disply is set up?
sensors fail, or just give false readings.
time of day,
alt barometric psi.
all can cause differnt readings. then add in heat. type of road did you just do a hard push in death valley, etc.
I bought a 10 tire unit since I pull a toad. http://amzn.to/2HjQPEB I would buy it again, support was awesome the one time i called after getting on the road and the over pressure alarms went off. I had set the upper pressure limits...(because i could), they quickly walked me thru putting just those back to default. And mentioned that I should leave the upper limits as default and only mess with the lower limits and maybe temperature settings.
I did not need them to setup the system as it was simple... It just should have mentioned the upper pressure limits were unimportant... who knew.
I would not drive without one of these anymore, they have saved my bacon twice now due to an inner dual valve leak. My tire repair dealer mentioned when I brought it in that most of the time the fiberglass all around the wheel well is gone due to them driving the RV on one tire as the inner was flat. The outer tire only holds up for a while at speed with double the weight its intended to hold.