I got bored on the motorway so tested the fuel economy at various constant speeds. All using cruise control so I could improve on all of these figures slightly by driving more efficiently over hills.
Even small hills make a huge difference (more noticeable than in previous cars, maybe because of the extra weight) so you need a longer distance to avoid getting a reading unfairly influenced by hills.
Couldn't do 60mph as I was worried about creating a hazard and my 90mph kept getting cut short so it's not as accurate. 70mph test was over at least 20 minutes and 80mph about 15 mins
70mph 45.2mpg 37.7mpUSg (113km/h 6.2l/100km)
80mph 39.0mpg 32.6mpUSg (129km/h 7.2l/100km)
90mph 34.0mpg 28.3mpUSg (145km/h 8.3l/100km)
I have run flats and staggered wheels so probably the least efficient combination. Tyre pressure at about +0.1/0.2 bar over recommended. BP Ultimate diesel in the tank.
My record average over about an 80 mile motorway run remains 52mpg (59mph average in heavy traffic) but that was driving like Miss Daisy and watching the mpg like a hawk
Might try and repeat this with different diesel but not the most scientific of tests!