The vacuum gauge is pretty much doing what the ScanGauge's instant mpg reporting from what I can understand. Which seems to be tied into the instantaneous throttle response. As rpm increases, vacuum increases as long as the throttle remains in the same position.
If there were some way to capture and graph the instantaneous readings then I would be able to match the gauge readings to roughly extrapolate my instant mpg. Over time I may be able to start guess-timating and adjust my driving to suit.
After driving around with my vacuum gauge installed I have noticed a few trends developing. Here are a few numbers for the curious observer:
Vacuum at idle is 18 in. Hg in warmer temps (13°C) with the engine fully warmed up.
The most common numbers during acceleration are 7 to 9 in. Hg...could be indicative of average mpg.
Exceedingly mild acceleration will yield 10 to 11 in. Hg, but I am subjected to dirty looks by drivers behind as they pull out and pass as soon as possible.
Cruising at 45 mph (65 km/h) seems to be yielding the best mpg - from 12 to 13 in. Hg on the gauge. Gauge will drop to 8 in. Hg on the mildest of inclines and idle or engine brake on the mildest of declines.
Wish I could do more pulse and glide driving, but there's too much traffic at most times.
The xB doesn't like lugging briskly, the Fuel Injection gets spastic if the engine is not reving in the 1500 to 2000 rpm range - the gearing is so low that the shifting time in each gear is only a few seconds and the computer has a hard time getting the settings right in that short of time so it feels mushy on the throttle response. I also notice that the MPG is pretty bad in the lower gears maybe they need breaking in more - the Geo would do the same thing usually the plugs needed gapping when that started happening. Best to keep the rpm low and sneak up on more throttle gradually and just wait for the speed to build. Adding a passenger yesterday and having damp weather sticky clutch and the seat a notch too far forward made for some really bad shifts and I really noticed the weight difference.