For many people, fuel efficiency is a primary consideration when purchasing a used or new car. These same people, however, often sabotage this efficiency by adding accessories to the outsides of their cars which create wind resistance. The harder an engine needs to work to push a vehicle through the air, the more fuel it uses. The degree of resistance to a smooth flow of air is called drag. How efficiently air flows around your car is termed its aerodynamics. The better the aerodynamics of a vehicle and the less drag, the better the vehicle's fuel mileage.
When practical, roll your windows up while driving. Driving with your windows down disturbs the air flow and increases drag. Keep in mind, however, that if your goal is to improve gas mileage, using the air conditioner will increase fuel consumption slightly.
When possible, remove unnecessary external accessories. These include rear spoilers (a spoiler actually increases drag and degrades aerodynamics), cosmetic air scoops, mud flaps, oversized side mirrors, oversized tires, extra lights, nerf bars, load bars and bike carriers. Any accessory that juts out from the body, even by a small amount, will increase drag. Some accessories you remove may leave noticeable holes in, or marks on, the body. An auto body specialist can remove them or cover them up.
A more aggressive approach is to lower the profile of your car. You can do this by installing shorter springs or by lowering your front grill. Shorter springs lower the ride height, reduce wind resistance in the front and improve the aerodynamics. Lowering the front grill minimises the amount of air that flows under the car, where air passing over uneven surfaces (suspension, exhaust, etc.) increases drag. Unless you have experience working on automobile suspension systems or auto body panels, hire a professional to install the springs or lower the grill.
A very aggressive approach is to install covers over unneeded openings on the outside of your car. This includes the rear wheel wells, which greatly disrupt the flow of air around your car. Unless you have the tools and experience to shape and install such covers yourself, however, hire a professional to perform the work. You can also install solid, smooth hub caps over your wheels. By covering the lug nuts and holes in the wheels, air can flow more smoothly past the wheels.
Though some of these aerodynamic improvements can be costly, others are free or quite inexpensive. Often, the money expended can be recouped in gas savings.
Several of these modifications require experience and specialised tools. Consult a professional in automotive maintenance or bodywork before attempting to lower your car, install an undercarriage panel or install covering panels. Panels must allow access for maintenance and for exhaust heat to escape.