I know going from 17s back to 15s my mpg went up about 5-7mpg. 17" wheels just weigh so much more than stockers. Plus like flatfourfan said more rubber on the road=more rolling friction. Also get an alignment and make sure you O2 sensor isn't bad. When was the last time you got a tune-up? Especially with 120k+ miles.

Oil change, fuel filter changem tranny fluid change, valve adjustment, timing adjustment.