I just went from stock 18" rims to 16" American Racing rims. I do not notice much of a change in my MPG. Like Tri said, make sure you get a rim with a large hub opening and the right back spacing/offset. Also, since you are going down to a 16" wheel, do a test fit on the front end and make sure the rims clear the brake calipers.

Then you are good to go!

Bart