These days calculating the mileage of your car has become simple and easy with the help of your odometer.
Fill the fuel tank of your car till the rim and reset your odometer to the zero point. Now, do a drive that represents your normal activity,
while ensuring it's significantly long enough. After, that fill your tank again till the brim, while noting the number of liters that you have tanked.
You can now calculate your car's mileage (or fuel economy) by dividing the number of kilometers that you have driven by the number of liters of fuel that filled the second time.
The result is the "kilometer per litre" or mileage of your car.