The Dual Mass flywheels in the 3.2 diesels seem to be fond of breaking and causing the owners lots of expense,see if you can get one that has had the solid mass conversion.
And like what was said above,the powerband in a diesel is from 1800 to 3500 rpm,so cruising is best at between 1800 and 2500 rpm,whereas in a petrol it is much higher,the power only really starts at 2500,but goes a lot higher than diesel. Theres not an equation that says: capacity * rpm = mpg If that is what you were wondering about