This is showing my age, but in the 1970's there were kits to install vacuum gauges in the dash. I never did (which means I'm probably not up to installing a MIMA on my Insight
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The thumbnail of how it works is when you cruise, you attempt to maintain speed with the highest vacuum.
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Vacuum gauge
See Figures 7 and 8
Vacuum gauges are always mechanical types that measure manifold pressure (engine vacuum), which relates directly to fuel consumption. Engine vacuum varies inversely with engine speed, so you should also drive at the highest indicated vacuum. Try to maintain the highest vacuum under all conditions.
The vacuum gauge readings are also a good indication of the condition of your engine. Actually, the readings are not as important as a steady needle. At idle, the vacuum gauge should show a steady reading of anywhere from 8-16 in. Hg (27-54 kPa) on an engine in good tune and operating condition. A needle that twitches at idle indicates fouled plugs, stuck or worn valves. A low reading at idle that stays low usually means a leaking vacuum hose, incorrect ignition timing or worn valves or valve guides. As engine speed increases, erratic readings may mean a blown head gasket or worn valves.
After the fuel crises of the 70's, the public's interest in fuel economy has given birth to a variation of the vacuum gauge, called a "motor minder". This gauge is a vacuum gauge with words ("Poor", "Fair," "Good" and "Excellent") and color bands (red/yellow/green) replacing the numbers on the face of the vacuum gauge. Since there is a direct correlation between in./Hg (vacuum) and fuel economy, gauge manufacturers have already interpreted the numbers for the driver.
The latest variant to the vacuum gauge, or the "motor minder" mentioned above is the upshift light. The operating principle is basically the same, there is a sensor monitoring the intake manifold vacuum which then triggers the light telling the driver when to shift to obtain better fuel economy