TAS increases when Weight decreases
 

In the Cessna 172S POH, a note to the cruise performance tables says that you may add 1kt per 150LBS below MTOW.
There's no such note in the 182RG POH.
So I was wondering if I could derive that information from the performance tables.
For different RPM/Manifold settings, and altitudes, the table gives the %Power and the TAS.
I could try to write the equations of Lift, Drag, and Power. From the tables I think I could get a sort of Cx as a function of Cz, and then at the end get my answer.

As I don't want to reinvent the wheel, I would like to know:
Is there a simple rule of thumb everybody knows but me?
Do my ideas to solve the problem make any sense at all?
As anyone any link to a well documented solution?
Thank you all for your answers

Long time I don't fly one of those...

What I remember is that in those airplanes, performance is usually referred to a fixed weight, typically MTOW. They don't bother in giving multiple tables for different weight ranges.

For the same power setting, TAS should increase when weight decreases. According to your 172 POH, this is at the rate of 1 kt per each 150 lb decrease. In the other one there's nothing written, Right? It can be either because they forgot to add the note or because the effect is negligible in that airplane.

Just guessing

In fact for any practical purpose, the effect is always negligible in a small plane.

My aim in asking the question is rather to increase my knowledge of flight mechanics.

hmmm

but I don't know what do you want to know, exactly.