Mods to get better MPG are numerous, but they don't have a gigantic effect. Best to do a bunch of small things like air up your tires. Blocking the upper part of your grill will give you some better aerodynamics. You will not overheat as you still have your lower grill open and your cooling system is designed to allow your car to drive through a desert with the A/C running. New Jersey just isn't that warm, so your cooling duct can be smaller.