I ride almost 100% on the motorway and even if I cruise 85-90 I can still get 52-55mpg (typicaly about 15-16l after 185miles - 2 days commuting). This is with a top box and a kreigs R35 rucksack which do seem to help reduce drag (increasing fuel effeciency by about 10%). Sub 80 does help and if I keep to nearer 70mph I'd almost certainly get over 60mpg.
I do always get much better economy off the motorway however, hardly surprising if you're usually running the bike at 5-5.5k rpm range.

The bike does seem to loosen up with age however, mine has done 12k miles now.