To calculate how many kilometers per liter you are getting in your car, divide the number of miles driven by the number of liters used. If you've travelled 140 kilometers on 11 liters of gasoline, you divide 140 by 11 to reach 12.73 kilometers per liter.
To find these two numbers, make note of the number of kilometers on your car's odometer the next time you stop for gasoline. The next time you fill up, subtract the first number from the current number on the odometer. This will be the number of kilometers travelled which you will then divide by the number of liters it takes to fill your car's tank.