Wind can make a heck of a difference. I've seen my mileage - on an RT - vary by 20+ mpg depending on if I had a head- or tail-wind. And an RT is more aerodynamic than the GS; my guess is that the cd of a GS+Rider is close to .9: only marginally better than pushing a steel plate thru the air
Another good argument for carrying a couple of Sigg bottles full of fuel. Glad you made it to the Shell station.