Are you talking about an accessory that physically adds on, like cruise control? There are aftermarket meters available like the Scangauge 2, which plug into the ODB2 port under the dash.
http://www.scangauge.com/. These will give you real time feedback about gas mileage as well as a bunch of other things that you might want to monitor as you drive.
I think more likely you might be referring to the "ECO" light on the gauge cluster, which as far as I know, only lights up when you are driving in an efficient gear. I don't know how helpful that really is, because it's such a blunt measure of gas mileage. I can see that there is an ECO light on my 2008 gauge cluster, but it never lights up. I think it was enabled in 2010.
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2008 Liberty Sport 4x4 Automatic
Jeepin By Al 4" Lift Kit
BFG KO2’s 245/75R16
Full MOPAR skid plates
Rock Lizard Skink Super Sliders
AtTheHelm Front Recovery System & Roof RackM116A3 Trailer build thread: viewtopic.php?f=72&t=77997