As you know, i was doing some research for my new closed loop 750cc conversion i was running there with and was doing full tank to tank tests using photos to record mileage, so heres my data:
We reset the counter at the fuel station as we left Blackpool and brimmed the tank to max....
Then upon arrival near Folkestone we found a fuel station and brimmed her again...
Channel tunnel was reached on £50 (49.5litres) of fuel and was exactly 331miles from our house. So travelling MPG in this cossie at English Motorway speeds is approx
31mpg. 
All in all, we used 346.49 litres and paid around £346 for it.
Divided by 4.5 we get around 77 gallons and we travelled 1659 miles giving us a figure of
21.5MPG overall.
The above figure of course included 21 laps of the Nurburgring and many hours of high speed cruising through Germany, Belgium and France
Not bad overall IMO.