Measure in L/100km as everyone has a different idea on how much "a tank" is, and if you haven't run out of fuel, you haven't used a full tank

Fill up the car, hit the trip meter, drive around until its close to empty, fill it up again, note how many liters you put in, and how many km you have done. Then work out how many L/100km you get.
I consistently get 8-9L/100km in mine with 215/75/R15 tyres.