EGTs function in this way:
Generally, if your engine is lugging and you are pulling loads higher than your engines output it will increase your EGTs.  This can be created with larger tires, lack of proper axle gearing, incorrect engine timing, altitude, and a slew of other reasons.  Higher EGTs are directly caused by incomplete burning of the fuel which also causes black smoke.

To reduce your EGTs, you can turbo charge (which makes a huge difference),
turn down your fuel, make other modifications such as propane injection, or do a tune up which is designed to allow your fuel to burn more completely.

Top recommend exhaust temps are in the area of 1300 degrees F before your turbo and 1100 degrees F after your turbo.  With higher temperatures, you risk possible damage to your engine head, pistons, and pre chambers if your engine is indirect injected.
 
 

The Gauge

The "Pyrometer" end of the gauge.