Hi, I'm new (whatup!), but I kind of did this calculation before buying my BMW (was choosing 330i vs 330d - went 330i...)
You need to compare the difference in fuel costs per mile (multiplied up by some mileage), to the difference of the non-fuel running costs.
To get the difference in fuel cost per mile:
Take the difference in MPG figures, divide by 4.54 to get difference of mileage per litre.
Take the difference of fuel cost per litre, in pence, divide that by the answer above.
This gives difference in cost per mile, in pence.
Example:
From Autotrader listings, the e46 330i manual gets 31mpg combined, The 330d manual gets 42mpg combined.
So; (42 - 31) / 4.54 = 2.423 (i.e. 330d does 2.4 miles more per litre of fuel)
From my local petrol station; unleaded 130.9, diesel 138.9 (so not yet 10p more here, also premium petrol will massively change the comparison!)
So; (138.9 - 130.9) / 2.423 = 3.302 (i.e. 330d is 3.3 pence per mile cheaper for fuel)
So if you take your 'additional non-fuel cost' for the diesel (I'm going to take £500 for an example), turn that cost into pence, and divide by the saving per mile on fuel.
So; (500 * 100) / 3.302 = 15,412 miles to break even, more miles makes the diesel the more cost effective choice.
I guess you could take a 'nothing major goes wrong' cost, and figure the mileage for that, then whack on a 'turbo blows' addition, and see how many miles extra means even that cost makes the diesel cheaper?
You have to remember to figure in all the differences in running cost when making that other figure though; road tax and insurance should be cheaper, but then you have the issue of the initial cost of the diesel being higher, or the costs being the same but the diesel therefore likely having many more miles on it...
Like I said; I did this calculation when buying my 330, and stopped roughly at the point where I realised the additional cost of the actual purchase of a similar diesel was likely to take me 4+ years to recoup on fuel savings - given that I average ~2 years with each car, and was buying a 2000 era car anyway...forget that!
Since this is my first post; I'd like to say a big thanks to the people on this forum; I read a LOT while researching the purchase, and you were all a really great source of info and advice (not that I asked any direct questions, but I read a lot of replies to questions I also had!) - especially Daily Doggy, actually, so its cool that my first post is a reply to thread of yours...