I find 4th gear anywhere from 40-65 to be stupidly economical whilst cruising, bit confusing as at 60mph it averages way over 50, rarely dropping below 48/49 and sits at about 2700 rpm - but 5th or 6th gear at lower revs at 60-80mph struggle to get over 50 unless doing so for a long journey (ie longer than 15 miles on the motorway)
as for miles per tank, 500 is around average which includes an odd blast now and then - 550+ has been seen when money is tight
. Thats on both Vpower and 'normal' diesel from tesco/esso/wherever. However, Vpower seems to be more smooth in the power delivery from low revs and also as soon as I fill up it reads over 500 miles in the tank, but on normal diesel it is only 400-430