Your current tire (175/60 R12) is 4.1% smaller than stock tire (145/80 R12). (%age is measured in circumference of the tire diameter)
When your odometer is Showing speed of 100 Km/Hr, you are actually driving at 96 Km/hR
This also means that you are actually covering less distance on ground but your odo is recording more Kms.
Now the dilemma
You are already traveling under speed, even then your car is suffering from mileage. It is suggested that your tire is creating too much drag for your car hence consuming more engine power/fuel.
And/OR your your engine needs complete tunning.
One more thing, due to bad gear box design, Alto tends to consume more on low gears, and its fuel consumption increases consequently
i)as u switch to AC,
ii)Fully load your car
iii) Any obstacle / load
IV) Big tires
(For this reason I haven't changed my stock tires yet. I will go for 155/75 R12 if changed)