To answer the original question, have a look at the aero/rolling resistance / fuel consumption tool I posted:
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
The default values you see when you load the page are for a 95+ Metro hatch, but you can enter your vehicle's values if you want.
You can see that aero & rolling drag are pretty much equal at 30 mph/50 km/h.
But even at just 20 mph, aero drag still represents 1/3 of the total.
Remember the comments on this subject from the developers of the GM Volt:
Quote:
|
One of the big surprises to emerge from the Volt's aero development has been identifying the extent to which wind drag affects the relatively low speed city driving cycle. In an environment where economy is measured in fractions of a mile per gallon, GM is finding major city mileage gains with its dynamic aero refinements on the Volt. - source
|
(That said, GM should be embarrassed to admit being "surprised" to learn this. Maybe their marketing people were "surprised", but the engineers weren't.)