Why does the EGT gauge in 172s have no numbers on it?
The simple EGT gauges do not have numerical scales because you never lean to a temperature.  You find the peak temperature, set the movable red needle to that temperature, and then lean to x degrees lean (or enrichen to x degrees) of peak temperature.  

Set the peak temperature at full throttle, full rich at a sea level airport.  

This system eliminates the concerns of calibration: Is the gauge calibrated, did the mechanic install the probe in exactly the right spot, etc. etc.