How to Calculate Fuel Consumption
Updated: October 31, 2019
With the price of fuels going up all the time, more and more drivers are aware of how much fuel their car needs. While the exact fuel consumption of your car changes based on your circumstances (city or highway, road conditions, tire pressure, etc.), finding out your car's fuel consumption is actually very easy.
Finding Fuel Consumption

Know that the equation for fuel consumption is "Miles Driven divided by Amount of Gas Used." A car's fuel consumption is a measure of miles driven per gallon of gas. If you know the distance you drove and how many gallons fit in your tank, you can simply divide the miles by the gas to get your "miles per gallon," or mpg.
 You can perform the same calculation with kilometers and liters as well.
 The best time to record is right after you fill your car with gas.

Reset the "trip odometer" after you fill up your tank. Newer cars have a trip odometer that you can set to zero at any time. It is usually on the dashboard or center console, with a small button you can hold to reset it to zero. Set it to zero when you fill up the car and check it when you need to fill up again  this is your mileage since you last bought gas.
 Your trip odometer will say "0 miles."
 If you don't have a trip odometer, record the number of miles on your car as "Starting Mileage." For example, if your car has 10,000 miles on it when you fill your tank, write "10,000."

Record the miles on the trip odometer right before you buy more gas. Before you start filling up your car at the gas station, record the mileage on the odometer as "Final Mileage."
 If you do not have a trip odometer, subtract your "Starting Mileage" from your current mileage to find out how far your traveled. If your odometer now says 10,250 for example, subtract 10,000. You drove 250 miles on that tank of gas.

Drive your car until the tank is almost empty. You can perform this calculation no matter how much gas is left in the tank, but the more gas you use the more accurate your reading will be.

Record the amount of gas you buy in gallons. Refill your tank completely and note how many gallons/liters you needed to fill the tank back up. This is you "Fuel Usage."
 You must refill your tank completely for this to work, otherwise you don't know how much gas your car used since your last tank.

Divide mileage by fuel usage to see your car's fuel consumption. This tells you how many miles you drove per gallon of gas. For example, if you drove 335 miles before refueling, and you filled your car up with 12 gallons of gas, your fuel consumption was 27.9 miles per gallon, or mpg (335 miles / 12 gallon = 27.9 mpg).
 If you measured in kilometers and liters, you should instead divide fuel used by kilometers traveled and multiply the result by 100 to get "liters per 100 kilometers".
 You have to start from a full tank and return to a full tank to know exactly how much gas your car consumed.

Practice calculating with an example. Terry's odometer reads 23,500 with a full tank. After driving for a few days he needs to buy gas. The odometer reads 23,889, and it takes 12.5 gallons to refill his tank. What was his fuel consumption?
 Fuel Consumption = (Final Mileage  Starting Mileage) / Fuel Usage
 Fuel Consumption = (23,889mi  23,500mi) / 12.5 gallons
 Fuel Consumption = 389mi / 12.5 gallons
 Fuel Consumption = 31.1 mpg
Finding Average Fuel Consumption

Remember that fuel consumption changes based on your driving. 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.

Record multiple tanks of gas in a row to find your average 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 trip odometer to zero with a full tank of gas. 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.

Record how many gallons of gas you purchase each time you fill up. 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.

Drive for normally for several weeks. Do not reset your trip odometer as you drive. Make sure your fill up your car 34 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.

Fill up your tank fully after 23 weeks. When you are ready to calculate your fuel consumption, top your car off and record the number of gallons you put in.

Add up the number of gallons you bought. 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.

Divide total miles by total 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).

Know that your car's advertised mileage is often overestimated. 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.
Minimizing Fuel Consumption

Avoid using the air conditioner. The AC uses gasoline to cool your car, which means you have less gasoline to actually drive with. Turn down the AC or turn it off once the car is cool to make your car more efficient.
 Running AC on max can decrease your fuel economy by almost 25%.

Drive at the speed limit. The faster you drive your car the more fuel you will consume. This is not a small change, either  every 5 miles per hour your drive over 50mph is the equivalent of paying $0.19 more for each gallon of gas.

Drive defensively. It takes more energy to start moving a car then to keep it moving. That means if you are constantly tailgating people, stopping and starting, or trying to pass, you're using far more fuel than if you were keeping an even pace.
 Try not to brake or accelerate harshly. Brake early instead of slamming on the pedal.

Use cruisecontrol for long, flat stretches. Cruise control will keep your car at a consistent and even speed, which avoids burning up fuel unnecessarily with small accelerations and stops.

Turn off your car in traffic. Idling, or leaving your car on when it isn't moving, wastes gas without moving you anywhere. When possible, cut the engine to save on precious gasoline.

Avoid rooftop cargo carriers. These greatly diminish your cars aerodynamics, slowing your car down and causing you to use more fuel. In general, towing trailers or loading up the trunk is a more fuel efficient option.

Keep your tires wellinflated. Underinflated tires can lower gas mileage by 0.3% if all four tires are low. Using the free air pump at most gas stations, inflate the tires up to the PSI recommended in your owner's manual.
 Some cars list the proper tire pressure on a sticker in the driver's side door or the glove compartment.

Replace your air filter. This is one of the cheapest and easiest ways to bump up your fuel efficiency. Make sure you buy the right filter for your car by bringing the make, model, and year in to your local auto shop and asking for recommendationsevery car needs a different filter.
 For newer cars, replacing the air filter will not help fuel efficiency very much. It will, however, make it easier for your car to accelerate without issues.
Tips
 Observe the posted speed limit.
 Don't speed up and slow down irregularly, it wastes fuel, especially in big SUV/Sedan type vehicles.
 There's usually no point in turning off your car at stoplights, since you usually only need to wait no more than 2 minutes if there's not much traffic. Only turn off your car if you're absolutely certain that it's better to turn it off (e.g. waiting to pick up someone at the airport).
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