Why price per liter alone doesn't decide the winner
Diesel and petrol prices differ, but so does how far a given vehicle travels per liter of each fuel — diesel engines commonly deliver better mileage than equivalent petrol engines. Because monthly running cost depends on both the price per liter and the mileage, the cheaper-per-liter fuel isn't automatically the cheaper-to-run choice; the mileage difference has to be factored in too.
The formula
Monthly cost for each fuel = (monthly distance ÷ mileage in that fuel) × price per liter for that fuel. Dividing distance by mileage gives the liters consumed that month; multiplying by the price per liter converts that into a rupee cost. The same formula applies independently to diesel and to petrol, then the two monthly costs are compared directly.
A worked example
For 1,500 km of monthly driving, a diesel vehicle at ₹90/liter and 20 km/liter costs ₹6,750/month to run (1,500 ÷ 20 × 90). A petrol vehicle at ₹100/liter and 15 km/liter costs ₹10,000/month (1,500 ÷ 15 × 100). Diesel saves ₹3,250/month here, or ₹39,000/year — even though diesel is also the cheaper fuel per liter in this example, the mileage advantage (20 km/L versus 15 km/L) contributes independently to the total savings.
Why the comparison can flip with different inputs
Because two independent factors (price per liter and mileage) drive each fuel's monthly cost, the winner isn't fixed — a petrol vehicle with a large enough mileage advantage over a comparable diesel vehicle could still come out cheaper to run overall, even while paying more per liter. This is exactly why the comparison needs actual numbers for a specific vehicle pair rather than a general rule like "diesel is always cheaper."
What this comparison doesn't include
This running-cost comparison covers only fuel expense — it doesn't account for the typically higher upfront purchase price of diesel vehicles, differences in maintenance cost, insurance, or resale value, all of which factor into the full cost of ownership alongside the fuel savings calculated here.