How do you lean mixture using only the instruments on a C172 Skyhawk? POH manuals always tells to use the voice of the engine as a reference. (Which complicates things a little bit since carbs and fuel injected planes have different "reaction times" for each cylinder) 

But how do you lean the mixture without using your ears? How do you lean mixture with only the instruments as a guide?

Lets assume we have a brand new C172 Skyhawk
The procedure varies depending on the engine instruments in the panel. Here’s an good explanation using the EGT probe on a Cessna 172.

And here are the manual pages from a Cessna 172SP that explains it.

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