On my BMW it is linked to the 'puter and is a real mpg gauge. Of course it is an instant gauge and not an average. It is tied to the road speed, RPM,and load and is not a vacuum gauge. When you come to a stop and are idling it goes to 0.
I use it to TRY to stay on the high side of the 20's

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
Fritzi '84 BMW
'92 Subaru
'96 Impala SS
'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad