Re:  Given the anomalies with the reported 65 mph data ...

Some may recall in the 1980s that VW Golfs (and others) had an "upshift
light" on the dash. (Maybe it was the '70s in which case it was "Rabbit.")
A prod to do just that and useful for at least two reasons:

1.  An engine running twice the rpms as another is very nearly equivalent to
a twice as large engine running the same speed as the smaller engine.  Since
gasoline engines mostly run near stoichiometric fuel/air ratio, it's nearly
a direct relationship with fuel consumption increasing with engine speed, as
the wider throttle opening permits consumption of more fuel/air mixture.

2.  AND ... any throttled engine consumes power just running itself by
virtue of the effort required to "suck" air past a partially closed
throttle.  Called pumping loss.  The upshift at low rpms usually results in
a wider open throttle condition, also increasing fuel economy.

Lots of interesting contrasts here with a diesel engine as they change
speeds not by increasing air/fuel quantity but rather by simply burning more
fuel to the same amount of air---yes, diesels don't have throttles and
concurrently don't suffer pumping losses as a result.  At whichever speed a
gasoline engine is running the fuel/air ratio is nearly always near 15 air/1
fuel, while a diesel could run at idle at near 150 air to 1 fuel  (helping
explain why an idling diesel cools down).

And, indeed greater speed equals greater drag, but at the same speed the
gasoline engine running at lower rpm (higher gear) quite obviously burns
less fuel.

Kent Christensen
Albuquerque
'81 Vanagon (owned since '83)
and '07 Merc GL320CDI and '08 BMW R1200RT, '02 BMW R1100S, '84 BMW R80G/S