When the warning light shows up, pa-gas ka. Full tank, automatic. Reset trip meter to zero then use the car normally. Forget the distance to km, ave fuel consumption etc., when the light shows up again, pa-gas ka ulit. Full tank, automatic. Look at your trip meter, divide what you see there by the number of liters fueled up. That should be your average fuel consumption. If it shows less than 8km/L the least, you're eating too much gas. If it's around 9, should be reasonable especially if you travel at noon and encounter traffic.

Hth