How long does it take to recover cost of a diesel car?

First thing first. This post is not about recovering the overall cost of a diesel car, but the incremental cost paid for diesel model over a petrol model of the same car.

I drive a 2015 VW Vento which has a 1.5 litre turbocharged diesel engine. Although I don't remember exactly now but I believe I paid a premium of ₹1.5L for diesel engine. At the time I had a commute of approx. 80 kms per workday. Recently, 6 years and 112,500 kms later, I came across a post on my favourite automotive forum, team-bhp.com i.e. Maruti now says, Diesel car costs can only be recovered after 260,000 km. Link to the post here.

According to India's largest car manufacturer, I still have 150,000 kms left to recover the premium I paid for my diesel car. I don't think so. I prepare financial models for a living, so I know how numbers can be massaged to prove any point. But how do you calculate the payback for a diesel engine. I don't know about Maruti's model; this is how I would calculate the payback for a diesel engine:

(on Jan 19, 2020, Delhi)

Cost of petrol: ₹85.20 per litre

Cost of diesel: ₹75.38 per litre

Mileage of petrol Vento: 12 kmpl

Mileage of diesel Vento: 18 kmpl

Driving cost of petrol: ₹7.1 per km

Driving cost of diesel: ₹4.2 per km

Diesel saving per km = ₹2.9

Payback period: 150,000 / 2.9 ≈ 51,750 kms

But there's interest cost involved in the ₹1.5L premium I paid at the time of buying the car. Assuming a rate of interest of 9% p.a. for auto loan, the saving of ₹2.9 would reduce by 9%.

Year 1 savings = ₹2.9 per km

Year 2 savings = ₹2.7 per km

Year 3 savings = ₹2.4 per km

Year 4 savings = ₹2.2 per km

Year 5 savings = ₹2.1 per km

Year 6 savings = ₹1.9 per km

Assuming I drive approx. 20,000 kms per year. My annual savings would be:

Year 1: 20,000 x ₹2.9 = ₹58,000

Year 2: 20,000 x ₹2.7 = ₹54,000, cumulative = ₹112,000

Year 3: 20,000 x ₹2.4 = ₹48,000, cumulative = ₹160,000 (cumulative ₹150,000 at 15,850 kms)

Hence, I would recover my premium at 55,850 kms.

Finally, a disclaimer: people say diesel cars cost more to maintain but I have not experienced any diesel exclusive expense in the last 6 years. A complete history of my maintenance expenses is listed in this post (Volkswagen Vento : Test Drive & Review). If there is any diesel exclusive expense, I would argue that it's counteracted by longer range and torque of a diesel engine.