Dear friends.
Fuel consumption is dependent on many factors and for a small engine car like 660cc u must fulfill all or most of them on regular basis.
Size of tyres. 145/80 R13 is the size for japanese alto 660cc i have 2014 model
Tyre pressure. Many people dont care about tyre pressure and if it is even 27 psi instead of 30 u will get huge difference in average. Replace nozzles or oversize tires if theres a problem.
For japanese cars most people dont know how to use the average meter. It is best to get the car full and reset the average meter and trip meter. On next tank full note trip meter and average meter and liters of fuel. Verify your average reading of car with manual calculation. If both are same u will know if your car is telling u the truth.
Select a good petrol pump and fill fuel from the same pump consistently that will help in better average
No. Of persons sitting in car affects its average drastically for 660cc cars. If there are a couple of people then its good but for 3,4,5 people u should get at least 1000 cc car. Cars give best average on low rpm. If u sit three or four ppl in 660cc car the wngine will continuously need higher rpm for acceleration and average will be lower. I drove on careem and got average of 6-7 kmpl lower compared to when i drive alone.
For 660cc cars and alto especially the catlytic converter gets choked which spoils average. It can lower your average by 4-5 kmpl while driving with 2 passengers. U have to get it cleaned with acid and ot will be good. (Handle it with care as it is sensitive.)
Nature of duty. Many people travel in city areas only with signals n rush. In this scenario your alto will have almost no edge on larger engine cars. But if your running is in outskirts of city like i go to raiwind from lahore then u can get the best average on light foot driving.
My car is driven 65000 km and gives me 25 kmpl while driving alone in summers with full time AC. In winters it gives around 30 kmpl with all the above factors satisfied.
U can say that with every factor unsatisfied u have to decrease your average by 3-4 kmpl. Starting from 25, if u dont satisfy one, u will get 21 kmpl, if u dont satisfy three u will get 15 kmpl. And thats when ur engine, tuning, oil change, air filter, etc are all good.
I hope now most people can justify their fuel average. For those who cant invitation to ask questions is open. 
