An example printed -
BMW 318i ES, Price £21,465 Economy 47mpg Price per litre 91.5
BMW 318d ES, Price £24,180 Economy 60mpg Price per litre 100.0
Extra cost of diesel model £2,715
Saving per 1,000 miles £12.74
Miles to break even 213,188


Ok some of the figures are not really real world (mpg) but you get the general idea.
Looks like petrol is the wise choice nowadays, what do you think?