RR Sport TDV6 versus petrol

[900ssduke]

Im thinking of changing out my Grand Cherokee V8 for a s/h RR Sport. I would like to use less fuel than I have been.(about 20 mpg) I would love the TDV8, but dont want to stretch to the money, so toss up between the TDV6 and the petrol.
What miles to the gallon can I expect from the TDV6, and is the performance ok. Im not looking for boy racer stuff, I just dont want sluggish.
What about the petrol, what mpg have people been getting?

[900ssduke]

Gazzab said:

TDV6 and a remap.

How would a remap affect mpg
regards