Compare the cost of running a petrol car against a diesel car
With the high cost of Diesel don't assume you will be better off changing your car to a Diesel.
Use my calculator see how long it would take to recoup the cost.
Calculate your dMPG
Because Diesel costs considerably more than Petrol, your effective MPG in a petrol car is higher. (When I originally wrote this application there was a 10ppl spread between petrol and diesel - that is no longer the case)
My BMW 540 recently returned 34MPG by driving very carefully, I much prefer wafting around in my V8 over a clattery old taxi-cab