Courtesy of Aopa:
A properly leaned engine yields the performance, range, and endurance figures found in the pilot’s operating handbook. If you plan a flight based on those figures and don’t lean the mixture, you can run out of fuel.
When you adjust the fuel/air mixture, you are decreasing fuel flow to the engine to compensate for decreased air density at higher altitudes. Less fuel is needed for proper engine operation as altitude is gained.
Always follow the recommendations in your airplane’s handbook. These steps will yield a mixture that is rich of peak exhaust gas temperature in a normally aspirated, carbureted engine.
It is known now that in Infinite Flight’s upcoming December 2019 update, 19.4, we will have the ability to control the mixture in the reworked Cessna 172 Skyhawk. Jason Roswell, Infinite Flight Staff, “you’ll be able to control the mixture, and use it to shut down at the end of your flight, along with mag checks!”. Now is the time to research what you need to in order to be able to properly fly the C172 in the sim. We have a bunch of educational content planned to do just that.