Well, the current price gap is Rs. 20 a litre on Petrol against Diesel
However, the recent announcements of hiking diesel price may further reduce the gap b/w Petrol and Diesel
Still for a fair estimate - keeping in view that one should buy a diesel car if can cover the initial payment cost in 4 yrs as benchmark. * then, ideally your monthly running should be bare minimum 1500 kms
How is this Mileage calculated
Diesel cars generally have better mileage over Petrol. How much !! Well - It generally has 25% - 30% better fuel economy against counterpart Petrol Car
So - if you drive 1500 kms a month - on a car like Swift, in city conditions you may get mileage of 15-16 Kmpl on Petrol and 18-20 Kmpl on Diesel
So - taking a rough estimate - would require 100 Lit in Petrol and 75-80 Lit of Diesel
On an average you will be saving Rs. 3000 approx a month on 1500 Km Drive on Diesel over Petrol.
It transforms to a saving of Rs. 1.25 Lakh in 4 yrs
So - if you feel that you may be driving atleast 50km (dailly) or 1500 kms a month for next 4 yrs - then it could be wise decision to move towards Diesel car, else Petrol should be the default option.
a) Diesel cars involve higher insurance premium year on year against Petrol cars
b) The Maintenance would be slightly higher in Diesel Cars against Petrol