the guage reads intake manifold pressure. when the car is off it reads atmospheric pressure in the intake manifold, 0 in. of vacuum, and 0 psi boost, when it goes clock wise, into the yellow, thats boost or PSI as it would be known IN THE INTAKE MANIFOLD. when the car is idleing and the guage is in the black part (counterclockwise) thats engine vacuum (no boost is present in the intake manifold) if you want exact readings you need a guage with numbers like an autometer or VDO boost guage. just rember that guage in the dash only reads Intake manifold pressure, not anything else, and is completly oblivious to altitide.