Its not so much your wheel choice that will affect you mpg, it's your tire choice. An F-150 with 20s would look bada**, but dont tell me your getting rockstars.
Depends on what the actual overall diameter change is and the width. Take in consideration of weight change and it all adds up. I run aftermarket 18 inch rims with wrangler tires in the winter and 22 inch rims in the summer. Each are about the same width and overall diameter but I get better mileage with the 22's due to they are acutally lighter by weight.