Aerodynamics matter at all speeds. Be it 10-20-30 and so on. Ask any soap box derby racer if aerodynamics matter during his or her coasting race? BTW with just gravity they can hit speeds up to 21 mph on a 950 ft track.
The best fuel mileage will always be attained by running in the highest gear at the lowest possible engine speed RPM's with out lugging the engine.
Most vehicles have a sweet spot for fuel mileage between 40 and 45 mph. That is the point where in most cases wind resistance becomes a factor with the power needed to over come it starts to accelerate quickly as your speeds increase.
Here is a white paper from Cummings Diesel that goes over speed and aerodynamics vs fuel mileage. Yes it involves big rig trucks and trailers but the principles they talk about still apply across all classes of road vehicles. BTW it is a PDF when you get to the page.
Cummins_Secrets_of_Better_Fuel_Economy.pdf