New range rover v6 diesel fuel economy vs v8 diesel

[2.5pi]

I'm thinking of buying a new RR to use as a motorway cruiser, given it looks a tad big I'm curious as to how much better in the real world the economy is at at a brisk motorway gallop ie sub 3 figures but not hanging about too much. Previously I've had poor diesel economy against full fat petrol in similar usage in X5's, anyone out there with relevant usage?

Appreciate any help!