I have an 07 wee with approximately 18,000 miles and am an older rider. For most of the time I have lived in Texas and recently relocated to the Seattle area. In Texas I always got over 50 to 54 mpg, however, if I went to Big Bend and ran 80+mph, then mileage could/would dip below 50mpg. Note, back roads of Texas are generally 70 mph and people drive at least the speed limit and then some. Now for Seattle, I live in Everett and most back roads have a speed limit of 25-45 mph. Even the Washington freeways are only 60mph. Consequently, I have recorded 61mpg tanks here and have concluded that speeds play the major part in mpg; not a new discovery.
Of course, there is also the thought that maybe the cold wet weather of the northwet is responsible for the better mileage but I doubt it.