In my '07 GXP I would get a displayed avg of about 26mpg, city and highway. About 30 on the highway.
After the GMPP turbo upgrade, I saw a slight increase. That was on a trip to Maine, well over 100 miles.
In my '08 GXP, I see a displayed avg of 26.5mpg city and highway. About 31 on the highway.
I don't expect the displayed numbers to be gospel but I am gratified to see they are so comparable between the two cars.
Of course, I can make the mileage much worse than those numbers, but I am traveling about 75 on the highway. Both of my GXPs seemed to get worse mileage at 70 than they did at 75, in 5th gear (manual).
Driving and working on a Sly 2008 Solstice GXP 5spd 260/260, and the worst-built 2012 LS3 Camaro 1SS to escape from Oshawa.
Gone but not forgotten:
2007 Solstice GXP 5 speed 290/340
Stored and needing to part ways with a 1970 Skylark Custom convertible I have owned since 1989 and turned into a GS455 clone. 462 with 7/8 primary headers, adjustable roller rockers, TA Performance Stage 1 aluminum heads, auto, Chevy 12 bolt, 3.31:1. Sadly a project car now but a driver from '89 to '06.