It could... it "frees" up power so you engine does not have to work as hard, same thing with an intake. but the MPG increase will not be anywhere close to how much you spent.
but the best mod will be your foot. drive closer to 65MPH, don't use the AC below 50MPH roll the windows down instead. check your tire pressure and go with skinny tires next replacement. do regular maintenance, oil changes, spark plugs, etc. lowering may help a little bit in aerodynamics....
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)