It's not a TRUE mpg gauge but rather it gives you an MPG estimate based on how much vacuum is used while driving.To calibrate the mpg, you have to fill the fuel tank, drive it on the highway for a good ways (at least 25-30 miles for my van since it'll get about 10-12 mpg), fill it up again... and divide the miles driven by how many gallons it takes to fill it back up. You then take that MPG number you calculated and set the MPG number on the gauge to roughly where it stays when driving at highway speeds.
So if the gauge sits around 15 for vacuum at 60mph and gets 11mpg, I would set the MPG part of the gauge at "11" to line up with the "15" of the vacuum portion of the gauge. The colored "low, good, bad, excellent" refers to how much vacuum there is, not what kind of MPG you are getting. The outer white ring of the gauge is the MPG portion.
A vacuum gauge is a nice diagnostic tool to have anyway since it can tell you what engine problem you may have based on how the needle behaves. So it's a nice little gadget to have.
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