The best setting is whatever gives you the best miles per gallon. You
should find that at around 40% power. The more drag the airframe has
the less it will matter, you'll find yourself saving an extra tenth of a
mpg. Not worth the effort. The 182 I used to have would indicate 135
mph in the summer at 23" and 2450 rpm and 13 gph(10.4 mpg). At 20
squared(about 45%) it would indicate about 100 mph while burning about
9(11.1 mpg). My Bonanza indicates 190 mph at 23/2500 burning 14.5
gph(13.1 mpg). At 19/2100(also 45%) I indicate 160 mph, burning 8(20
mpg). Using true airspeeds would change these slightly but you get the
idea.

john wrote:

Shortly after getting my private license I went to a different FBO and
got checked out in their aircrafts - C172 and Piper Cherokee.

I talked with the CFI and asked what RPM settings he would recommend.
I'm renting dry, so one way to reduce cost is to slow down and thus
save fuel. At the present time I'm just building hours towards my
commercial ticket. Rarely am I in a hurry to get anywhere. He
mentioned that if I needed to get somewhere quickly, to keep it around
2400. If I'm just building time then to reduce to 2300. It will save
fuel as well as reduce the sound level.

The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.

Thanks,

John