If you read the manual, you will find it states the following: (C172N)
En Route Climb
1. Airspeed -- 70-85 KIAS
2. Throttle -- FULL OPEN
3. Mixture -- RICH (above 3000ft, LEAN to obtain maximum RPM)
So there's your answer
C172R (fuel injected, so potentially different to C172N)
En Route Climb
The mixture should be full rich below 3000 feet and may be leaned above 3000 feet for smoother operation, or to obtain maximum RPM.