noah wrote:

Basically - is it bad for the aircraft (C172) to float around at
around 85kts with the power somewhere below the green arc (say
1800rpm) and the Carb-Heat on? Ok, there is a second benefit - my club
charges based on tach time, and I have to be honest about that -
although I really do wish for the tours to be slower.

No problem, but, as Peter says, lean the engine.

George Patterson
If you're not part of the solution, you can make a lot of money prolonging
the problem.