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For example, stopping and starting the car a lot uses much more gas than driving at a constant pace. This is why highway consumption is always less than city consumption.  Cruise control can help you get better fuel consumption. Fuel consumption gets worse the faster you drive. Since AC uses gasoline, using it will diminish your fuel consumption. To get a more accurate picture of your car's fuel consumption, you need to have more data. By driving for longer and averaging your fuel consumption you eliminate "glitches" in your data. Say, for example, that you calculated your fuel consumption one day as you drove up into the mountains. Because climbing uphill takes more fuel, your fuel consumption would look much lower than it normally is. Set your odometer to zero and don't reset it after you get a tank of gas. If you don't have an odometer, record how many miles are on your car with a full tank of gas. To get a more accurate measure of fuel consumption, you need to know how much gas you are using. Each time you fill up, write down the number of gallons you purchase and save it. Do not reset your trip odometer as you drive. Make sure your fill up your car 3-4 times for an accurate reading. Try to do this during a month of average driving, as big trips or unexpected traffic will change your fuel consumption. You do not need to fill your car all the way up each time. As long as you record the number of gallons you put in you can calculate fuel consumption. When you are ready to calculate your fuel consumption, top your car off and record the number of gallons you put in. This represents the total gas used over this period of time. If I bought three tanks of gas, 12 gallons, 3 gallons, and 10 gallons, then my total gas usage would be 25 gallons. Use your trip odometer to see how many miles you traveled total, then divide this by gallons to get your average fuel consumption. While this is the exact number of miles per gallon during your test period, it is a good estimate for your car's average fuel consumption. For example, if you used 25 gallons of gas, and drove 500 miles during that time, then your average fuel consumption would be 20 miles per gallon (500 miles / 25 gallons = 20 mpg). By law, car makers must post the average fuel consumption for cars. However, these are only estimates, and they are frequently on the high end. You can look up your car's fuel consumption online through this US Department of Energy website, but to find your car's actual miles per gallon you'll have to calculate it yourself. If your calculation is drastically different from the suggest average, you may need to bring your car to a mechanic.
Remember that fuel consumption changes based on your driving. Record multiple tanks of gas in a row to find your average fuel consumption. Set your trip odometer to zero with a full tank of gas. Record how many gallons of gas you purchase each time you fill up. Drive for normally for several weeks. Fill up your tank fully after 2-3 weeks. Add up the number of gallons you bought. Divide total miles by total gallons. Know that your car's advertised mileage is often overestimated.