There's something real wrong with 7mpg. with a G.C. While they're not the MPG champion of the world, there could be a multitude of issues causing the poor performance. It could be as simple as a brake caliper or two dragging, with low tire pressure, to any number of engine/transmission issues. I agree with J.E.S., get it checked out.

that's a 3 ton car overall about 7 is not bad , should be 12 but it's also 8 years old, it's only rated as 15/20 high way new and those figures were wildly optimistic, change air filter

I'd have a diagnostic run on it and see if that tells you something. Sparkplugs are the first, best thing to try. Ours gets 19 on the highway.

Autozone