When the car is at idle it should read 20-25 inhgs once you start driving depending on how much you put your foot in it the gauge should read close to 0 at 0 or Positive pressure anywhere from 10-20ish depending on how much you put your foot down. The boost gauge should only show boost when you're accelerating at a certain rate. It's about throttle position and RPM.How do you "read" the boost pressure gauge ? What is a normal reading at idle ? During acceleration ? While cruising at 70-80 mph ?