Yep - I've found the best way to be 'economical' with the FF is boot it and get it up to speed then stay at that speed for as long as possible. I managed to average 37 (according to the on-board computer) on a trip down from the Highlands. I have no idea how since we had 5 people in the car, a dog, 6 boulders (yes granite boulders), a 5ft pillar drill and everyone's luggage from a long weekend at my grandpa's 90th. We came down cutting through Yorkshire and then picked up the M1. Topped up in Stirling and was on ~ 1/3rd of a tank when I filled up again in Kingston.