Like Ropal, I have been tracking mileage since I bought the K1200GT in June 2007. His numbers agree with mine - the OBC mileage is normally 2-3 mpg higher than actual. Actual mileage is computed by dividing odometer miles by gallons at the pump.
Here are tricks that I have learned to increase mileage:
1. Remove the side cases and any topcase that you may have. Leave the luggage behind.
2. Keep the windscreen on the lowest setting and tuck in tightly. Put your handlebars on the lowest setting, and keep your head down low, under the slipstream if you can. Wear gear that doesn't flap.
3. Use something other than California fuel formulations, the highest octane you can find.
4. Keep your speed down. This is just a guess, but I bet that the optimum speed is between 45 and 65 mph.
5. Keep your speed steady - wide open roads are best. Use the cruise control to maintain speed discipline. Stop and go riding will kill your mileage.
6. Keep your RPMs low, but don't lug the engine. Lugging will ruin your mileage, too.
7. Ride at higher altitudes. I don't know exactly why this helps, but it makes a big difference.
8. Ride in cool, but not cold weather.
9. Ride with the wind at your back.
10. Start at a high altitude with a very long descent. Downhill is a great mileage booster.
(I am thinking about a route that starts high in the Rockies, and descends for 200 miles or so.)