Chris ] gurgled happily, sounding much like they were
saying:

I would like to make a graph of car fuel consumption versus speed. My
driving is not smooth enough to gather the raw data myself. Do you know
of any reliable figures, or graphs?

They don't exist.

I'm interested in relative values, rather than those for any particular
car.

They'll vary widely between classes of car - although individual cars
within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.