Yes, aerodynamics make the biggest difference in FE at higher speeds.
In lower speed driving with traffic control devices (AKA stoplights), adjusting the nut behind the wheel makes a bigger difference than aerodynamic mods, IMPE. (YMMV)
With the exception of 3 road trip fill-ups for my Mustang and one road trip fill-up for "Big Red" (with slower driving), my commuting MPG is better than my road trip MPG. But that's OK for me. I drive a LOT more miles commuting than I drive on road trips.
