Why is my throttle 3/4 closed at cruise?

Last week I flew my Grob from San Jose (RHV) to Torrance (TOA)
and back. Total time was nearly 6 hours, but burned
only about 30 gallons of fuel. The book says 9 gallons per
hour at 75% cruise. I've noticed this before, that it never
burns nearly as much fuel as I would expect. Another thing I
noticed during the flight that I had never thought much about
before is that in cruise, the throttle is about 3/4 closed, even
at 2600 RPM at 7500 feet MSL. Redline is 2700, and opening the
throttle further would push past redline. I haven't flown any
other planes in a while, so I can't recall whether this
throttle position is unusual.

All this seems to say that I am cruising around at Best Economy
(which the book says is 45%, 5.3 gal/hr). This would also explain
why I get only 105 kts IAS, rather than 115-120. I would
surely like to go faster, but I can't figure out how without
exceeding redline RPM. Is my tach in error? Do I need a different
prop? (fixed pitch) What else should I be checking? The engine
is a Lycoming O-320, same as a Warrior.


Happy landings,
Ross Oliver