Your picture is of an "Economy Gauge" Basicly a vacume gauge. The Ideal was if the driver "Romped & Stomped" it would show poor milage and at steady cruise would be in the green - good econmy range.

http://s237.photobucket.com/albums/ff251/mbigd73/72%20gmc/?action=view&current=72gmc017.jpg

Gm had some factory installed usually in large sedans & station wagons as another gauge on the dash. This one appears to be a add on later.
The couple I have driven were a pain... jump all over the place. Until you get used to them the needle kept catching my eye something moving will.

Hope this helps
Brad

http://s320.photobucket.com/albums/nn343/brsdb3381/Tuesday_MY_56_Chevy/

56 3100 6 cyl