After all the MPG naysayery, I thought I'd do a bit of testing on the way home.
This is the result of sitting at exactly an indicated 70 (about 67 in reality as verified by GPS) for the 7-mile stretch of M3, staying at this speed constantly. This photo was taken just as I was approaching the M25 junction after about 7 miles. The counter was reset just as I got up to speed after joining the motorway. The MPG rose to about 37.5 after joining the M25 and sitting at 55-60mph for a mile and a bit, then promptly dropped to about 32 as I hoofed it all the way through 2nd and 3rd gear when coming off the M25 onto the local dual carriageway
. I then drove the ~2.5 miles through town with a bit of stop-start traffic before arriving at home with it showing 32.1MPG.
I didn't do anything special, I don't drive like a granny at 10mph below every speed limit, I don't slipstream lorries. I just keep my car well-maintained with the right tyre pressures and whatnot, drive gently, keeping up with traffic whilst looking ahead and anticipating what's happening.
I normally sit at 70-75 on the motorway but thought I'd rigorously stick to an indicated 70 for the purposes of this.