get a full tank of gas, then reset your trip meter to zero. on your next trip to a service station, take note of the present trip meter reading, as well as the # of liters refilled.
tama si ssaloon. remember, you NEED to fill up to a full tank, and only until the automatic shut-off level of the pump (wag mong ipa-sagad hanggang fuel filler neck) para siguradong pare-pareho lagi yung starting level mo sa tangke.
better to counter check the trip computer figures against actual pump liters though... minsan inaccurate yung trip computer readings for liters of fuel used.