Improving MPG

Your driving style could have a lot to do with your mileage most cars never see there advertised mileage. It will be hard to increase from you cars stock mileage, there a lot of great ideas on here already, but you want to keep cost down or otherwise its pointless. Unless your looking for performance with mileage. Comparing Civics and Neons is a bit unfair if you look at cars with similar weight and aerodynamics mileage hasn't improved a lot with 40 years and billions of $$$ especially in town cause it just takes a certain amount of fuel to move your car down the road. EFI was mainly an emission move by automotive industry, and overdrives are more of a performance thing cause you can gear a car for mileage or performance or a compromise of both where OD can give you both.

weight(car and/or drivetrain), aero, getting air in and out of the engine easier, less accessories etc..