Back-of-envelope calculation.
My last petrol car was a Vauxhall Omega 2.5 V6 and I used to get about 25mpg on a run.
Current car is a Volvo V70 2.4D and I get about 40mpg on a run (46 mpg going to Gloucester last Saturday).
I have covered 160,000 miles in the Volvo since I bought it in 2008.
At 40 mpg that's 4000 gals of fuel, say £5 per gallon = £20,000.
Omega woukld have used 160,000/25 =6,400 gals, petrol/diesel roughly same price=£32,000.
Savings on diesel = £12,000. Enough said.