The less air/fuel pumped through the engine per mile, the more miles per gallon. A low manifold pressure (higher vacuum) means less fuel per revolution but, fewer revolutions per mile at the same manifold pressure also means less fuel per mile.
This is why Corvettes and Vipers (among others) are geared far beyond their power curve. The new 'Vettes will actually go faster in 5th gear than 6th because they are over geared in the interest of better fuel economy.
Chrysler had this figured out back in the days of the Mobil Oil annual economy runs. They would start off quicker than competing drivers who were going strictly by manifold pressure, even spinning their wheels occasionally, in order to get to top gear as quickly as practical.
