I have put Splash Guards on all my Hondas. I have them for my '06 HCH along with the full mask. There hasn't been any affect on FE...mainly because I don't have the car yet.

2005 Honda Odyssey EX-L RES & NAV - Desert Rock Metallic

2006 Honda Civic Hybrid NAV - Magnetic Pearl

2007 Toyota FJ Cruiser - Black Cherry (First Toyota, Third non-Honda)