Assalamualikum Everyone. Dear Brother, As per your query and per my personal experience of the Altis 1.6, you will get no more than 8 to 10 km/l with a/c depending on your foot aka pedal to the metal. Yes, added considerations of your motor oil type, tyre type, rim type, etc will add some changes to your car drive and mileage. On long drives expect 13 to 15 km/l. This is my personal experience and yes adding some hi octane will add some pleasure and smoothness to your drive but may not effect fuel economy that much. I drove a friends GLi 1.3 and noted a days drive average of 11 km/l with ac on normal petrol. Drove the car on full tank and next day refueled the car to see mileage. Just my 2 cents.
But honestly the drive pleasure and quietness of the altis 1.6 is way better. Just off the record, I have sat with an uncle on the way to hyderabad from karachi and noted and average of 15 km/l on a grande 1.8 altis and the smoothness of that engine with the same ride comfort of the 1.6, does have a added bonus. In other words the CVTi shifts and manages the fuel economy better. I would suggest getting the Altis 1.6 for better drive pleasure. If you can afford it get the CVTi 1.8, otherwise the 1.6 will be a better choice in the long run. Last and worse case scenario get the 1.3 GLi manual (10-13km/l) as the Automatic variant is not the best of fuel economy (7-10km/l) class cars, and the pickup is so so, but will get the work done.