[xs2man]

You can't take the absolute difference in purchase costs into account when you do these calculations though. When its time to sell the car, the diesel will still be worth more than the petrol, so you will recoup a lot of that cost at that stage.

And even then, you are not taking into account service costs. A 2.0 TDI will be cheaper to service than the 325i because it requires less oil, and will be covered by fixed price servicing, which the 325 wont be.

In reality, your £5k-6k isn't the real number you should be using either. Presumably that number comes from trading in your car against another. Which isn't the actual difference of the two models worth. The actual difference in that case would be better made between the selling prices of the two models, as trading in you are always going to loose out, regardless of if you were to go to a petrol or diesel. The bulk of that cost can be associated with your want / need to change, and shouldn't be taken into consideration in these calculations.

I did have some serious calculations to do before I bought my 335d, because my 2.0 TDI A4 was so terrible on fuel. It costs the same per mile in fuel as the 335d costs. The cost to change wasn't too bad, but that was the cost because I wanted to change. I also had to trade in a 3 month old motor for a 2 year old motor, a huge difference in miles (3500 against 17000), but better spec (slightly) in the BMW.

The only real way you can determine the difference between petrol and diesel is against actual fuel consumption figures. No other calculations should really be factored in. At a push you could pop tax differences in there, but they will be minimal compared to fuel costs anyway, so not really worth even considering.