Nige Danton laid this down on his screen :
I'm a cyclist and aerodynamic friction plays a huge role in
determining speed and above ~30kph the benefits of drafting behind
another cyclist are considerable. There's an energy saving of ~20% for
the first cyclist in a pace line and that rises to maximum of ~30% for
the fourth cyclist.

I really would be surprised in cars are so slippery that aerodynamic
friction does not play a significant role at speeds slower than 60mph.

I fancy it will be a trade off in a car between the gear in use and
aerodynamic friction. Going just fast enough to be able to stay in top
gear will be the most economic speed in most cars, which will be
slightly below 60mph.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk