I recently purchased a 1986 245 DL from my neighbor. He bought the car through the tourist & diplomat sales program, and therefore custom ordered the car. Anyhow, he got four gauges installed. A small tach, a voltmeter, a oil temp gauge and a vacuum gauge. That's what it says in the original purchase literature (I'm lucky enough to have all of the records.) The vacuum gauge is funky. It seems like a real-time mpg gauge. The needle jumps when I floor it, etc. I just wanted to know how it worked. Direct link to the accelerator pedal or what? And why the name "vacuum? "

Too bad you don't have a turbo. They are really entertaining. Not only vacuum, but boost too. Merrily driving out and about, or as we say in Canada: Ooot and Aboot, showing lots of vacuum and good mileage. But wait - there's a pesky driver trying not to be last in line and wanting to jam infront of you, nevermine there must be a mile of roadway between you and the next car behind. Ok, so you jam the gas pedal down, the vacuum disappears only to be replaced by lovely wondefull pressure. Not too much though, your don't want too much pressure lest your internal engine parts scramble themselves to bits! The car leaps forward closing the offending gap. Hey, cool you're not last anymore. Only a slight decrease in fuel economy.

Hey, I get 26 - 28 mpg out on the highway.

Not from personal experience, you know.
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Warren Bain - '86 744Ti M46 >282Kmi, '96 965 >97Kmi Wifemobile near Manassas Va

Vacuum gauges suck big time.

When your engine is running, it mimics a large air pump. The intake side always runs at a vacuum, and this vacuum is modulated by the throttle position. When you floor it, more air is admitted into the intake manifold and the pressure rises to approach atmospheric -- thus, vacuum disappears or is reduced.

Your greatest fuel economy occurs when you maintain the highest vacuum, which corresponds to the lightest throttle.

Idle should be around 24-25, or so.

You're right. It's totally useless!

Noooooo.........

A vacuum gauge is a suckometer.

They're quite useful for monitoring driving habits. (But, then, so's your collection of speeding tickets.) Vac gauges can also warn you of burned valves, poor timing, and other engine problems. I believe they will also tell you of a plugging cat.

Of course, you need to know how to read the subtle clues....

If you think that gauge is useless, I'll happily take it off your hands....

I didn't say it wasn't fun to watch.