> to keep it against such small things as air friction isn't all that large.

At 75 mph air friction isn't small.  A lot of small cars give up not much 
faster than this because their engine cannot generate enough power to 
overcome the air friction.

>  Compare it to the amount of fuel needed to get the car from 0 to 100 km/h
> in a relatively short amount of time.

Much less than getting to 100 km/h in a long time...

IMO, the surprise at the small amount of fuel needed comes from the fact 
that the energy density of petrol is so high.  We hear figues like 6 litres 
per 100 km, but 6 litres of petrol equals around 200 MJ of energy, enough to 
lift a 747 plane 100 metres in the air!

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