One thing to take into consideration: the wide variations in mpg we're
seeing here could very well be related to odometer variations in these 20+
year old vehicles.  Bentley Section 90.30 tells us that, brand new, odometer
tolerance is from -1% to + 3.75%, in other words, a spread of almost 5%.
Couple that with over- or under-sized tires, and you could get variations of
20% easy.

I've noticed that going taking the exact same route from our home to our
vacation place yields different mileage readings on the odo. And I mean
exact same route! Maybe five miles difference over the course of 400 miles.

Claims of driving X MPH for such and such a MPG may be erroneous, too. It
turns out the speedometers have a total tolerance, new, of almost 18%!
(Bentley 90.30) Incredibly, VW indicates that for a real 50 MPH speed,
speedometer readings can be up to 58.7 MPH! In other words, when you think
you're going 75, you may only be doing 65 or so--and that's with absolutely
stock tires.

What are we really measuring, then? This is all ballpark figuring, when you
look at it that way. EPA must calculate MPG using measured distances, rather
than odometer readings, along with carefully measured gas.

I was originally asking a mechanical question. Is there something that
typically cuts gas mileage down even though the car runs fine? What would
that be, if so?