what should be the average monthly drive to buy diesel or petrol car

Hello Aditya,

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.

* Note
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