How to calculate fuel consumption.

To calculate your vehicle's actual fuel consumption you need to know two numbers: the distance traveled and the amount of fuel used to travel that distance.
Here is one easy way to calculate fuel consumption:

- Fill up your tank to the first click-off and reset the trip odometer to zero.
- Drive.
- Next time you come to a gas station for a fill-up, write down your trip odometer readings, fill the tank the same way, to the first click-off and write down the amount of fuel it took.
Now you've got what you need - the amount of fuel used and the distance travelled between fill-ups. To calculate your MPG, divide the total number of miles driven between fill-ups by the number of gallons used:

MPG = miles driven / gallons used.

For example: You filled up your tank and zeroed the trip odometer. Next time you at the gas station, your trip odometer shows 320 miles and it took 18.5 gallons to fill up the tank again. That means your car consumed 18.5 gallons to drive 320 miles. So the calculation is:

320 miles / 18.5 gallons = 17.3 MPG

For a Metric System, you need to multiply the number of liters consumed by 100 and then divide by the kilometers driven.
The formula is:

L/100 km = number of Liters x 100 / number of kilometers.

For example: You filled up your tank and zeroed the trip odometer. Next time you at the gas station, your trip odometer shows 480km and it took 42 Liters to fill up the tank again. That means your car consumed 42 liters to drive 470 kilometers, or its fuel economy is:

42L x 100 / 470 = 8.9 L/100 km

Once you know what your car's actual fuel economy is, you may want to compare it to what it should be.
You can find an estimated fuel economy rating for your car at: Fueleconomy.gov.
If your actual fuel economy is significantly worse than estimated numbers, this could be an indication of some problem with your car.

Poor gas mileage causes

You probably already know some common causes such as excessive speed, jerky driving habits, extra load, roof rack and excessive idling. But what about mechanical problems?
There is a lot of possible vehicle problems that can cause increased fuel consumption. Often it's a combination of various problems. If your vehicle has poor fuel economy, may be it's time to do a tune up. Below are a few common examples of the car problems that can cause poor gas mileage. Of course there is a lot more issues that can affect fuel economy. If your vehicle has poor fuel economy, have it properly diagnosed to find the exact cause.

Under-inflated tires According to Fueleconomy.gov, under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. Faulty Oxygen Sensor A faulty or "lazy" oxygen sensor is a common reason for poor gas mileage. Your mechanic can check the oxygen sensor with a scan tool. Read more about an Oxygen sensor Leaking fuel injectors A leaking fuel injector can affect the gas mileage and cause the engine to sputter for a few seconds after start. Faulty Ignition components Faulty spark plugs, ignition wires, distributor cap, rotor, or ignition coil can cause the engine to misfire, which will affect the gas mileage. Plugged catalytic converter A plugged or restricted catalytic converter is another common culprit of many driveability problems and poor gas mileage. Your dealer can check it. Vacuum leaks Vacuum leaks also can cause driveability problems and bad gas mileage. Vacuum leaks can be caused by bad intake gaskets, rubbed-through vacuum hoses, damaged PCV valve, etc. Stretched timing chain or belt The timing belt or chain drives the engine camshaft or camshafts. Over time a timing chain or belt can stretch causing retarded cam timing. This can affect the engine performance and gas mileage. Old or improper grade engine oil If the engine oil hasn't been changed in a long time like in this car, it becomes thicker and can cause increased friction inside the engine. Similarly, the proper oil grade should be used. For example, according to Fueleconomy.gov, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1–2 percent. Stuck open thermostat The thermostat helps to warm up the engine faster and maintain the engine temperature within the normal range. Sometimes the thermostat can get stuck open. This can cause the engine to run at lower than normal temperature and use more gas. Read more about a Thermostat Faulty torque converter lock-up function If a lock-up function of the torque converter of the automatic transmission doesn't work for some reason, the fuel economy will suffer.

Under-inflated tires According to Fueleconomy.gov, under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires.