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I know this is an old post, but I'm catching up on the forum and wanted to weigh in.
Vacuum gauges are a great and simple item to add to your ecomodding. But you can't get too wrapped around numbers. 18-20 are good for idle, 3-5 for acceleration, 25+ for decel. Book values.
What it really provides is INSTANT feedback about where your throttle is. If you're accelerating from a stoplight at 10 in/Hg, ease your foot a bit at you'll get 12" and better economy with no noticeable speed differential. None of the electronic doodads give you that feedback that quick. A tach tells you how fast the crankshaft is turning, but has nothing to do with fuel consumption.
But the best part of a vac gauge, is the diagnostics. Is it low at idle (restricted exhaust?), have a regular drop (burnt valve?), wandering needle (carb probs?)
They are relatively cheap and easy to install. Initially ignore the numbers and just see where your car fits. Once you understand your engine's personality you can truly tweak it for max mpg
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