The windshield size is probably the biggest single physical variable. The RT won't get the mileage that the RS gets for a number of reasons. Riding style, loading, 2 up or single rider, even tire pressure will cause mileage to vary.
You can figure about a 10% increase in fuel mileage if you use non Ethanol fuel. I found that I consistanly got about 2 more miles per gallon if I used Premium fuel over Regular. This was in hot weather where fuel octane will make more difference than in colder weather.
As stated before, the Spyder is not a high fuel mileage machine. I think with a different engine configuration it could do better, but it will never be a gas sipper. The current powerplant is not ideal for this application. Our 990 (which it really isn't) is a re-engineered crotch rocket motor. It makes power with high RPM and does not make the torque we need, where we need it. Though BRP did a very good job of reshaping this motor for the Spyder, it just can't be what it is not. Being made for a very light, low wind resistance motorcycle, it just isn't as comfortable in the Spyder.
What the Spyder needs is a lower RPM, high torque (with about the same BHP) engine to realize it's full potential. Hopefully it will get that kind of power plant at some point. Then I think you will see better fuel mileage.
