Here's a better way to measure milage. Fill up your tank, when the pump stops wait a second and give it a squeeze until it stops again. Set your odo to 0. Drive around until you feel like filling it up again. Fill it up the exact same way each time. That amout of fuel that you now put in is how much 'petrol' it took to go whatever milage is displayed on your odo.
I just filled my tank and I got almost exactly 10 liters/100 km.
Not great but not bad either. How do you like that!