The RG fairing is way more aerodynamic than that barn door they call a batwing, I had a 2000 EG that I converted to a RG & the mileage improved by at least 2 mpg, better the faster you were going, absolutely no other changes
Yes, this is undoubtedly true. I put 36k miles on my 2016 EGC batwing, mostly highway riding 75 mph +, very little 'round town riding. The batwing fairing produced so much cockpit turbulence, after nearly two years on this bike I started looking for relief. One day, me and the wife on a whim took a 2017 RGU out for a test ride from Santa Fe HD. A twenty mile test ride showed us how superior the RG shark fairing is to the batwing. Greatly reduced vibratory turbulence for us both on the RGU. A few months later, I traded for the 2018 RGU, and am very happy with the airflow performance of the shark.
I have no problem believing, even in the absence of scientific evidence, that the features of the shark vs batwing that reduce turbulence also work to make the shark slice the air better, and therefore need less fuel to push the air.