I have been experimenting and have just done 442 miles on a tank after travelling newcastle chesterfield and then sellafield after brimming the tank and then getting down to 4 miles on range before i decided not to risk it any more to fill her up.
It was a mix of main ly motorwqay but also a66 and a595.
This in my 3.0 so coupe.
Its a great car, will be even better once i get the seat adjustment sorted, lots of tweaking to be done