[Deerhurst]

Not only can you help economy with a vacuum gauge but you can diagnose a whole lot of things with one. Your engine depends on vacuum. Rough running at idle, vacuum gauge has lower than normal vacuum? Check for vacuum leaks. Blip the throttle and it should go down and then when it comes back pass the normal idle point then return to normal. If it doesn't this can signify a different issue. If the needle wiggles around at idle you have either a huge cam or bad valve sealing. It's also fun to watch. My truck pulls about 22inhg and idle with a slight, barely visible, fluctuation due to my mild cam.