Fuel efficiency and slow driving

Need to look into this, but its a pretty good place to start:

From http://ask.slashdot.org/comments.pl?sid=993269&cid=25350317

Since most of these things don't change while we're driving, the proportionality can be expressed more simply:

F = kv^2

In other words, fluid drag is equal to some constant multiplied by speed squared.

The cube comes into it if you multiply both sides by v again:

Fv = kv^3

Because Fv is force times speed, which is just another way to write power. (Fv = Fd/t = E/t = P where d is distance, t is time, E is energy, and P is power.) Therefore the power dissipated by fluid drag (and thus the power required to maintain speed) is proportional to the cube of the speed.