You need to do the calculations for Golf 1.6 Petrol vs Diesel

This car runs on stock unleaded. According to my calculations

$2500 extra costs me 13% lost investment income in my share fund per annum = $325 per year.

Extra fuel consumption is 2 litres per 100km.

So, to even start paying off the $2500 cost, I have to save more than $325 per year in fuel.

You forgot to factor lost opportunity cost of the extra cost.

Also the fuel cost is now 15 cents cheaper for a 1.6 petrol vs diesel.

This equates to a 0.8 litre per 100km equivalent fuel saving at current prices.

Half of the saving in consumption is blown in extra fuel cost.

Even if I travel 20,000 km per year, I am still behind in cash terms and the $2500 outlay will never be paid off, no matter how long I keep the car.

Then you have the issues with dual mass flywheels that cost major dollars every few years etc.

And PRAY you never need to do major engine repairs on a diesel engine, as that is mega dollars.

And Pray you never get a shonky (added kero + heating oil) load of diesel or water in the fuel, as that will totally stuff your engine and require a full engine rebuild.