Get rid of any unnecessary weight in your car or truck, such as golf clubs or unnecessary tools and materials. Lowering the tailgate on a pickup truck actually reduces your gas mileage by reducing drag, too. If the tailgate isn't lowered, an air cushion is created behind the cab that creates a low drag surface for the air to travel over. But if you have to carry something, carry it in your trunk, not on the rack on top of your car. With something on top of your car, there is way more drag, slowing your car down, making it more difficult to accelerate. A dirty air filter will rob the engine of power and efficiency. It will use more gas than a clean filter. So if it's been a while, get it changed. It should only take a few minutes – in fact, you may be able to do it yourself. In the scheme of car parts to replace, air filters are pretty cheap. Most are about $30-40. Properly inflated tires will increase your traveling distance on a tank of gas. Keep a tire gauge handy and check the pressure. Tires naturally lose air over time, so don't be alarmed if one or more is low.  Check your tire pressure more often when the outside temperature changes greatly. Hot tires will have high pressure from expanding air – but if air is let out to the proper amount of pressure, then when cooled they will become too low in the cooler conditions, so you'll need to reinflate them when cooler. Under inflated and greatly over-sized tires have more rolling resistance and this will reduce your miles per gallon (MPG) and cause tire wear from the excessive friction. If you do need new tires, get radial ones. They have a low rolling resistance, keeping your car aerodynamic. It's a good idea for your fuel economy to change spark plugs and wires on schedule, and the coil(s) when their effectiveness is breaking down (such as shorting internally). The better your engine runs, the more power it will produce using less fuel. As a bonus, fewer oil changes are needed saving your time and reducing trips to the mechanic as well as less used oil for the environment. When you're getting your tune up, ask your mechanic about what oil you should be using for your car and for saving gas. He may be able to throw in a bottle or two with your tune-up. Before changing your tires to the modern style thick ones, think again. The more the width, the higher the rolling resistance offered and higher the fuel consumption.

Summary:
Make your car lighter. Change your air filter. Keep your tires inflated to the proper pressure. Keep your car tuned-up at manufacturer recommended intervals to optimize your miles per gallon. Use synthetic oil in your car to save on fuel consumption. Use stock tires.