In 1993 I bought a Geo Metro. For those unfamiliar this was a three
cylinder 1 liter displacement car that weighed in the neighborhood of 1700
lb. It naturally was using 1993 off the shelf technology and was fuel
injected.
It was a small car, almost scarily so at times in some traffic, and didn't
have a lot of power, though it could easily go 75 mph on the flat. Up hill
and against a very strong prairie wind (that is somewhat common here in the
dakotas) it could reach a terminal velocity of 65 mph. I think I had up to
80 mph at one time.
Most would say it was underpowered, but having grown up with VW beetles in
the 50's and 60's it didn't seem that bad to me. It was more powerful and
accelerated better etc, than the beetles of that era. It was a dream to
park and maneuver in big cities. Was cramped and had rather large amount of
road noise, although, again it was less than the old beetles.
I routinely got 55 mpg out of that vehicle on the interstates. And my
record was a few tanks that got 65 mpg (for some reason that I do not
understand this occured only on trips in Colorado and New Mexico). This
mileage was good enough to drive from St. Louis to Atlanta on one tank of
case (11 gallons) or for about $11 at the time.
I'd conclude from this that fleet averages (for passenger cars) could easily
be at this level. Of course, few wanted to buy the car. Interestingly
enough more recent models of the vehicle do not get as good gas mileage. My
guess being that they tweaked the motor in later years for power rather than
efficiency and also increased the weight of the vehicle by a bit.