Many peope OVER estimate the speed at which aero drag becomes a significant factor in fuel consumption.
Have a look at the
aero & rolling resistance calculator in the Tools section of the Garage (link is below the header, top of page). It will shed a lot of light on this issue.
E.g. the default values on that page are for my car (98 Firefly/Metro) in stock form. Notice the speed at which aero drag accounts for *half* of the fuel burned? 30 mph. Not insignificant. At 45 mph, over *70%* of fuel consumption is spent overcoming aero drag.
Drag reduction is worth pursuing unless all your driving is stop & go.
Another thought: The average speed in the Shell fuel economy competitions is probably in the neighbourhood of 20 mph... yet just look at how much effort goes to aero there.