Edmunds has the Top 10 Tips for Improving Your Fuel Economy.
Some of the simplest and cheapest things you can do is keep your tires properly inflated, keep your car clean and waxed, remove extra weight and drag from your vehicle, drive smoothly, turn off the AC and roll down your windows to cool off at slow speeds, avoid excessive idling, use cruise control for flat terrain, use the highest gear possible, combine your trips and slow down on the freeway.
Last but not least replace your air filter and spark plugs and get a complete tune-up at the recommended intervals.
Check your tire pressure - the proper pressure for your tires is in the owners manual or sticker on the driver's side door jamb. Proper inflation can reduce fuel consumption by 3% according to the Department of Energy (DOE). Besides the fuel savings you'll be safer driving a car with properly inflated tires, since under-inflation increases rolling resistance causing heat to build up and increasing the chance of tire failure.
Wax your car - having a shiny waxed vehicle makes it more aerodynamic - one person reported a 7% reduction in fuel consumption on a 1600 mile road trip.
Reduce weight - ditch those sand bags you put in the back of your pickup last winter for traction. If you have a roof rack, remove it unless you are using it - roof racks create drag.
Drive smoothly - avoid excessive acceleration (quick starts and stops). This saves fuel and wear and tear on your engine, transmission and brakes. Anticipate traffic flow - it's not going to matter how quick you get to the next red light in the over-all scheme of things...slow down relax. Don't tail gate people on the highway - you wear out your brakes and end up having to speed up and slow down which burns fuel.
Use cruise control selectively - you'll save fuel on flat roads but the cruise control will use more fuel in hilly areas.
Turn off the AC - and roll down your windows to cool off - this saves fuel at slow speeds. At highway speeds, driving with your windows down increases drag and fuel consumption.
Maintain your car - change your air filter and spark plugs, and get a complete tune-up by a mechanic, at the suggested intervals. A dirty air filter can increase fuel consumption by 10 percent. A faulty oxygen sensor can increase fuel consumption by up to 40 percent.
Avoid excessive idling - If you go to drive-thru's and have to wait awhile either shut off your engine while waiting or better yet get out of your car and go inside...or even better yet stop eating that stuff and start walking and biking to your local cooperative, farmers-market...or Safeway - whatever is in the area. If possible change your commute times if you have to deal with freeway gridlock so you commute during off-peak hours.
Use the highest gear possible - If you have a manual transmission try to get it in the highest gear possible as quickly as possible without lugging the engine. If the terrain and traffic allows it you might want to try coasting (or at least try to avoid riding your brakes while going down hill unless it's necessary to be safe).
Slow down - simply slowing down from 75 to around 55 to 60 mph can reduce fuel consumption by about 20 percent as shown on this U.S. Department of Energy graph.
