I do a 63 mile daily round trip to work, 60/40 motorway/B roads. I have an 07 Dooper.
Using cruise as much as possible I have experimented with different speeds and I have found that (on a 'non-windy' day) in my Dooper:
67/68 mph gets about 65-68mpg (I use this the most as you can pass lorries without dawdling! and the mpg is good)
60-65 mph around 70mpg.
70 mph about 58-59mpg.
75 mph anything from 52 - 55mpg
80 mph lucky to get 50!
The most I have got from one tank is 555 including miles left in the tank. I reckon 600 is possible, 640 on an 08 Dooper going on the better Mpg?
I have 16 inch bridge spokes and auto-air conditioning (that I don't use very often)
Also the Hi-Fi stereo, that if I'm travelling alone I have it LOUD..
I will test BP Ultimate and/or V-Power before it gets to £1.50/litre
(judging by the way that crude oil is having daily new highs, probably in a few months!
)