cheap/easy mpg boosters?

Lose as much weight as you can (Car not you:) ) and-aerodynamics. Front air dam and side dams will help. And a flat pan for the under carriage. spoiler in back will not help. Make sure you use a soft air dam to stop it from breaking on curb stops/parking. Get it as close to the ground as you can. Helps with cooling and handling at speed by making a low pressure air pocket under the car, Pulls cooling air through better and "glues" the car to the road. Yes it acts like you added weight to the car but the inertia will not increase and the lower air drag more than makes up for the downward force.