I have a F850 myself and drove about 30.000 km with it through the Middle East. It is a very comfortable bike. Best speed is between 80 and 100 km. You can drive for hours!
With respect to your question about fuel consumption: that depends on the type of bike (fuel injection vs carburator), the condition of your bike, the type of fuel and, of cours, your speed. I have a carburator engine and at a speed of 80 km it uses about 1 liter in 23 km or 4,5 liters in 100 km). That is minimal consumption. At a speed of 100 km it uses about 1 liter to 20 km. Beware: this bike needs good fuel. Below 95 octane you should use some octane booster. Keep the air filter clean! With a dirty air filter and bad fuel, the fuel consumption rises considerably.