My Logic:-
For this I am comparing Brio petrol and Ritz diesel, one of the best cars in their segments.
Brio:
efficiency-12(Full AC-city)
petrol price-75(in my city)
Amt per km=75/12=6.25 Rs
Ritz:
efficiency-19(Full AC-city)
diesel price-46(in my city)
Amt per km=46/19=2.42 Rs
Price difference between base models of both-Rs 130000
Running-800km/month(average)
One year savings=((800*6.25)-(800*2.42))*12=3064*12=36768
End of five years=Rs 183840
For someone like me who keeps his car for more than 10 yeras, diesel makes sense.
Other factors:
1) Diesel prices may adopt dual pricing method so car diesel prices may increase drastically making all my calculations baseless.
2) RESALE

-Swift and Ritz diesels will enjoy huuuuge resale value maybe even 1.5-2 L more at the end of many years.
3) Diesel will give you peace of mind. Many trips which you would avoid because of petrol will be made if you are running diesel.
4) Rs 130000 extra will generate 8% interest which I didnt calculate. But resale nullifies it.
4) AC Always ON !! Biggest luxury.
5) Petrol prices are only going to go further north. Remember that always !!
Conclusion-Actually either petrol or diesel will be OK for this running. But diesel rules for me because of peace of mind regarding hikes, AC always, Turbo

and well....I will just use the car more !!
A diesel car always increases your monthly running.
P.S. This calculation may be stupid....I dont have a lot of financial experience.
Actually I am confused between Brio SMT and Ritz LDi at the moment. Please comment on my calculation so that I can make an informed decision.