Have you ever noticed that when you fill up your car that the first half tank empties a lot quicker than the second half. This has actually been tested and does happen as the weight of a fuller tank of gas does actually reduce fuel economy just like unnecessary weight above. As you empty the tank and approach the bottom half or quarter you have lost the extra weight and get a better 1-2% increase in fuel economy. Its not a huge increase, but you can save about 3 to 6 cents per gallon just by filling your tank to the half mark instead of filling it up. Now this would not make sense for people that have to drive long distances where there are very few gas stations. Use your common sense here.

Another fuel economy buster is driving fast and over the speed limit. According to EPA estimates, for every 10 mph you drive over the speed limit you lose 5-10% in fuel economy or more (depending on the size of engine, car weight and current non speeding gas mileage). So if you like going 80 mph in a 60 mph zone you are losing upwards of 10 – 20% in your fuel economy (not to mention that you could get a speeding ticket which could easily greatly increase your car insurance premiums for the next year and cost over $300 in court fees, fines and etc...)