[elhigh]

Vacuum readings are hypermiler feedback. In general you want it high because it means you're demanding less of the engine, burning less fuel.

The comment about the diesel is entirely correct, and of course you also get the added bonus of the higher energy content of the fuel. But that's a big job, swapping in a diesel.

This is where pulse-and-glide comes into play, asking your engine to pull in the efficient but thirsty low vacuum mode, then relying on inertia to maintain speed for a while. But that big box won't coast really well when empty, and the '89 Econoline wasn't offered with a manual, so P&G is a lot more problematic.