Recently Ribbert told and showed us the tire pressure readout unit he uses.
If you remember, it used a sensor that was attached to the well of the rim, inside the tire.
This unit (below) uses sensors located in valve caps, certainly easier to install.
https://www.youtube.com/watch?v=sOLP5X5tBsU (https://www.youtube.com/watch?v=sOLP5X5tBsU)
Note: I have NOT used this unit, but I have purchased a number of other items from this seller over the past couple of years. I have had no problems with them.
Arnie