My g6 is currently at 1100 miles but I reset my fuel econ meter at 800. For that first 800, my meter read somewhere between 22-23 mpg. The vast majority of that was in town driving with speeds 35 mph and lower. I reset the meter at 800 miles because I was going on a 150 mile trip of highway driving and wanted to see what I would get on the hwy. For that trip the meter read just below 32 mpg.
I average about 19 in the city. On my 200+ mile highway treks I have gotten about 33 every time. These are according to the computer. I would say the actual values are somewhat lower. Over all I'm pleased with the fuel mileage, it is improved over my Grand Am GT.
Im used to floating gauges (my Z would slam you around in the seat and easily exert ~1G when pressed, parking on a hill, stopping hard, etc.. I know how it is 'normally').. this is much different. Its almost like its 'estimating' how much fuel you have??? Hard to explain.