Don't Carry More Than You Need
Keep the car as light as reasonably possible. For each 100 pounds of extra weight, gas mileage is reduced by as much as 4 percent. Limit the everyday items in your trunk or cargo area to the bare necessities, which should include some emergency items, such as a small jug of water, a flashlight, and a few tools.
Don't haul around what you won't be using. Leave the golf clubs at home until you head for the links. Not only does extra bulk add fuel-gulping weight, but it can also upset your vehicle's normal weight distribution. That will impair handling and can even rob a front-wheel-drive car of valuable traction. If you must carry heavy items, try to put only a few of them inside the car at a time.
Remove That Rack
Wind drag increases fuel consumption. Get rid of anything that disturbs the smooth flow of air over your vehicle's surface. Most roof racks have removable cross members, and some racks can be removed altogether; take off your car's rack if it isn't in use. When you do need to carry something on the roof, keep it light and small -- both for fuel-saving aerodynamics and to avoid the risk of a top-heavy weight imbalance.
That grille bar and those running boards may make your SUV look rugged, but they also add weight and drag. And that bolt-on trunk-lid spoiler that makes you feel fast and furious? It's designed to harness the wind and press your car to the pavement at high speeds. The result is better grip on the road, but this "downforce" is actually artificial weight that hurts fuel economy. Worse, unless you are a racing technician versed in aerodynamics, chances are excellent that your spoiler isn't doing anything more than adding wind drag and weight. That's costing you at the pump, too.
Retain That Tailgate
Some pickup-truck drivers take it as an article of faith that they're saving fuel by driving with the tailgate down, or removed, or replaced by a mesh fabric or metal gate. False.
Aerodynamic studies show a pickup truck is most fuel efficient with its tailgate up. It seems the upright tailgate causes air flowing over the roof of the cab to collect as a stagnant "dome" in the cargo bed. As speed builds, this dome, which tapers in a teardrop shape near the tailgate, acts as an aerodynamic ramp that forces airflow over the tailgate, to the benefit of fuel efficiency.
Disrupt this flow by dropping or removing the tailgate, and air coming over the cab is left to swirl around in the cargo bed, degrading the truck's aerodynamics and hurting fuel economy.