My last attempt at simplifying vacuum.
When the throttleplate(s) are closed, or near closed, such as when idling or cruising, the piston(s) still " suck " air through the engine.
Therefore = high vacuum ( negative pressure ) in the intake manifold.
When you open the throttle, all the air required by the engine goes " unrestricted ", and therefore low ( very little vacuum ) exists in the intake manifold.
I've never scrutinized the " economy gauge " on an M-B, but suspect it is nothing more than a vacuum gauge that reads " backwards ".
