Just been doing a comparison of sorts.
Anyway -
10,000 at 47mpg for a diesel at ?1.41 = Fuel cost of ?1363.47 ish.
(Approx 212.7 gallons which is 967 litres = 1363.47)
10,000 at 27mpg in a 180 at ?1.36 a litre = Fuel cost of ?2290.24 ish
(Approx 370.5 gallons which is 1684 litres = 2290.24)
Insurance - for me anyway -
Diesel = ?350
180 = ?450
Tax -
Diesel = ?130
180 = ?245
So a years driving a Diesel is roughly - ?926.77 cheaper on fuel and ?215 cheaper on tax and insurance
Total - ?1141.77 .........
Clearly as things go up - the diesel becomes even cheaper - but as mileage goes down so will the savings.