I've been consistantly at at 18 mpg since i bought the car.
It never gets less fun
Red 08 SS

 

ive been considering buying this car for awhile now...so its good to see about 90% of u are averaging 20+mpg...my jeep gets 15mpg on a good day taking it easy lol but i normally average 14mpg and its so slow i barely reach 70mph with ne wind at all lmao....so ill switch ne of u ne day lol

 

you'd probably have to leave cruise on 85+ to get less than 20mpg...lol
I tried and failed...21.1mpg

 

I've never gotten less than 24 MPG, even with a lot of local driving.

 

31.1 MPG on 1,051 highway miles on cruise control set at 71 MPH in an HHR SS Automatic, 2 full tanks of gas, 93 octane pure highway driving from NC to NJ and back,
POST GM Performance Tune upgrade.

All I can say is im very impresses, and mostlikely attribute it to a slightly the slightly higher boost levels (-2 to -5 psi) the car cruises at now after the GM Tune. More air mixed in with the same amount of fuel gives a bigger explosion with more horsepower. Kinda like Blacksmith 101, use the bellows to put more air in the fire to create a hotter fire!