1.
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.
 
2
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 you 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."
 
3
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.

4
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.
 
5
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.
 
6
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 your answer will be in kilometers per liter (kpl). Many Europeans multiply this answer by 100 to get "kilometers per 100 liters" of gas.
You have to start from a full tank and return to a full tank to know exactly how much gas your car consumed.
 
7
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