Ahhh. Big mistake.

Between your gauge and your engine is the one-way check valve (which may be leaking a bit...). The white cylinder thingy is a vacuum reservoir -- it collects vacuum (odd concept, to collect nothingness) for use actuating your various button-operated vents, etc. So your gauge is effectively reading the vacuum level in the reservoir.

This might tell you how well your defroster vent will operate, but it has little to do with gas mileage or engine performance.

Attach your gauge to a fitting directly on the intake manifold.
--
Don Foster (near Cape Cod, MA)