With my 1.8 Rabbit inline gas motor, I honestly do get 23-24 mpg and I
always have for about 39k miles of hard driving.  I keep track of my fuel
consumption rate as a way to monitor the tune of the motor. I faithfully, on
every fill up, divide the miles traveled by the amount of fuel using a
calculator right there at the pump.  Takes about 30 seconds.  If I were to
see a big change in my fuel consumption, I'd be looking for the problem.  So
far, my gas mileage has diminished less than 1 mile per gallon since I
installed my extra interior...So my motor IS getting a little worn, a tiny
bit less efficient, but not by much..But I bet, with a modern sequential
fuel injection system and a bit more sophisticated engine management it'd do
a heck of a lot better..
  I did once see 19 miles per gallon, humping through the eastern Sierras
with all my tools and camping gear aboard, bucking a very very stiff storm
wind..but even here in the Columbia River Gorge, where it's often VERY
windy,  not often does it ever use more than a gallon to go 21 miles and I
have seen  a tankfull give me 27mpg  once..All this is at my normal highway
speeds of 60-75 miles per hour.
  So for about what you'd likely pay for a set of tires, when my gas mileage
falls into the teens, I'll go to the wrecking yard and get another fresher
Rabbit motor...Or maybe by then someone will have an even more frugal
choice.
  In my original post, I was curious about motors like..oh, some of the
small Toyotas and Hondas that are around 100-150 hp and sip fuel at nearly
50miles per gallon.  One of my Canadian buddies just sold his diesel
vanagon, so slow it was scary but an honest 36 miles per gallon and replaced
it with some non-descript Toyota sedan, a few years old with 50 miles per
gallon fuel rating......
  What about motors like that, never mind the Hybrid technology..Those cars,
the hybrids, are for the wealthy, or at least for someone other than a
worker-bee like me..
  Don Hanson