So here is an interesting question.

Say your renting a plane thats charged per tach hour.

You have a 182. For 2100 vs 2400rpm you will drop 10kts in speed.
for a 650nm flight your almost 4.5 hours at 2400
for the same 650nm flight you might do it in 5 hours at 2100

Is the extra cost of that 30min of flight time off-set by the reduction of RPM and resulting tac-hours?

depends on the calibration of the tach. if its a 2310 tach (1 hour at 2310=1 hour on the tach) then 5 hours at 2100 should read about 4.5+/- on the hour meter. correct? and your 4.5 hour trip at 2400 will read 4.7+/- hours. a 2566 tach would change those #'s a bit

lets say your fuel burn at the lower RPM was 1gph less. total fuel used would be very close. if youve got an extra 1/2 hour why not try it & see.

I rented an almost brand new all glass 172 wet when they came out for one day (24 hours). The rate was from the hobs, and I don't remember the numbers, but it worked out for me to run it flat out. I had 11.5 hours when I returned it. They seemed "stunned" that I put so many hours on it.

If Mr Murphy has anything to do with it, you'll burn less but have to poop 1/2 an hour from your destination.

hahhhahahaa, thats why big-gulps exist!