I just read a big article on this the other day. In reality, due to the distance from the wing to the front of the RV, they don't do much good. They may even make things worse in regards to fuel economy. I always liked the looks of the polished stainless wings on top of the dually, but never have installed one.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.