On a 1000 mile trip up and down to Nick at Austin's did 22.5 out and 21 back (battle with a few Belgians). 90% motorway at the legal limit +5% on cruise. (Bit too much for the froggie-fuzz; must pay that euro-45 bill for 91kph instead of 90!).
... I guess I get about 15, no better, on my cold-engine start, 8mile+8mile daily commute in the winter. An average of 17-20 sounds about right for normal usage? Best I have EVER got was 25 on cruise at 60mph, sea-level, flat road, no passengers or luggage, air temp 15c., 990 mBar and around 90% humidity. Boring.
Better than my 5.3 XJ12L which was 1/1 (in litre/km terms = 2.35 mpg) on the 3 mile winter school-runs in the village ! Hummers are better!
David