My flying club has recently started charging out its 172's based on tach
time rather than Hobbes time. In my experience fixed pitched airplanes
I've rented have been charged out based on Hobbes, while constant speed
airplanes have been charged out based on tach.

So recently, a friend and I took a 172 to do some pattern work. We were
out a little over an hour according to the Hobbes meter and our
wris****ches, but the tach registered about half that. Now I've flown a
182 and noticed about a 10% to 15% difference between Hobbes and tach
time depending on what I was doing. Especially in the pattern, I never
noticed anything like a 50% difference. So I began to think about it.
With a fixed pitch prop, at a given throttle setting, rpms will drop in a
climb and increase in a descent. They also drop when the throttle is
pulled back. Hence our low tach time in the pattern -- lower than peak
rpms in the climb, then low rpms on the downwind and descent because of
the reduced throttle. In the 182, however, you have near or maximum rpms
in the climb and, with the propeller fully forward, higher rpms in the
pattern and descent than you would in a 172 with a similar throttle
setting. Hence the tach time should be higher in the 182 than in the 172
despite similar wear and tear on the airplane.

So, this got me thinking. Given a 172 charged out based on tach time,
what would be the most expensive way and the cheapest way to get from
point a to point b? Most expensive -- climb at Vy+5 knots, then cruise at
a TAS of 110 knots. Cheapest -- do a Vx climb to altitude (to keep the
rpms low); when at altitude, chop the power completely and start a
descent. After descending for a while, climb back to altitude at Vx.
Repeat until reaching destination (or sick, whichever comes first). Am I
crazy? Isn't this why fixed pitch equipped airplanes are usually charged
out based on Hobbes time?