Re: Increase mpg. Undertray?
An underplate definitely improves MPG, but the whole aerodynamics of the vehicle need to be designed in concert with it. I don't think just adding one is going to help you.
2014 Jeep Grand Cherokee Overland V6 4X4. Maximum Steel w/ Vesuvio Jeep Brown/Indigo Blue. Advanced Tech Pkg.
2013 Lexus GS350 AWD, Liquid Platinum w/ Flaxen. Luxury & Cold Weather Pkg
Previous Jeep: 2011 Grand Cherokee Laredo X w/Nav, QT1 & Trailer Tow