No expensive sensors, my TPNS is based on tire rotation speed.My car's TPMS does not read out pressure, only pressure change, so I too have a dial gauge in in the glove compartment.
Sometimes too much technology exceeds the benefit... I am an old fan of KISS.
I haven't shared this lately but there is merit (imo) to running higher tire pressure on the interstate where you would like lower rolling resistance and would be willing to trade off a little traction. However for lower speed driving off the interstate you want the softer ride, and full traction.
my low tech solution is a pump mechanism built into the flexing tire sidewall, so every tire rotation could capture a tiny bit of air to keep the tires filled. Of course too much pressure is dangerous so a pressure release valve would be built in too. Ideally there could be a centrifugal component to the pressure relief valve so it holds higher pressure at highway speeds than it holds at lower secondary road speed.
Ideally this would be invisible to the driver and just work.
JR
PS: This is probably difficult because I am sure I am not the only person to have this idea.
