Rolling resistance drag, which weight and tires impact, isn't a big portion of the overall drag at higher speeds. You'll see a larger impact during stop and go driving than highway is what I'm saying. It really depends how grippy and heavy of a wheel and tire combo you go with. Way too many variables to definitively say how much your mpg will be impacted.
You should choose a tire size that keeps the tire circumference the same or close to the same. A 225/40-18 (637.2mm) is roughly the same diameter as a 215/45-17 (625.3mm). Bumping down to a 35 series tire would decrease the diameter to 614.7mm on the 225 width, 18" compatible tire.
This is a good tool to determine fit.
http://www.frsproject.com/tire_size.php
Remember, size varies a little with brand and model of tire. More square shoulders on a tire will leave you with less clearance.