Always good to have hard numbers on hand if you want comparison's of expense rather than guess work.
15k miles x 3 years = 45kmiles
petrol cost @ say 30mpg => 45000/30 = 1500gallons = 6810 litres x 1.35p/litre = £9193.50
deisel cost @ say 45mpg => 45000/45 = 1000gallos = 4540 litres x 1.42p/litre = £6446.8
Difference = £2746.7 = £916/year = £76.3/month
Insurance saving(your numbers?) = £1200-£550 = £650
ignoring tax variation, saving potential = 650 + 916 = £1566/yr
costs, this needs more reasearch to increase accuracy/validity
say car value halves from 6k to 3k, thus cost = loan cost (say 6.5k at over 8%apr) - remainder car value/ 3 yr = (6500-3000)/3 = £1167/year (conservative?)
ok, so ignoring tax/servicing and selling of mk3- total saving 1566-1167 = £399/yr
ok, So maybe you should have done this ages ago, it looks like it will pay for itself!!
