2006 2.4L loss of mpg
I have a 2006 G6 2.4 with about 113,000 miles on it. the last time I took it in to a "quick oil change" place they tried to sell me on a fuel system cleaning. Sounded like a hoax to me and I passed. The problem is that not long after (maybe a day or two) I noticed my mpg displayed on the dash dropped by about 3-4mpg and the drop was almost instantaneous. It was not uncommon for me to get 34 mpg on the interstate and 21 or 22 mpg in town if I was careful. Now I'm lucky to get 31 mpg on the interstate and about 19 mpg in town.
I haven't had any indicator lights come on to tell me something is wrong and the only reason I know that the mileage has dropped is because I almost always leave the display set to watch the mpg ratings. It doesn't feel any different driving, just lower mileage rating.
Any ideas what might be causing this? From what I've seen this is roughly the estimated MPG, but I know that I was getting better not long ago and want to find a way to get it back.