When should I use high gear?

User Lemursurp on Reddit explains the use of High Gear, stating:

It's for mid-range torque. It's useful for attaining and maintaining speed on flat and uphill paved or hard dirt roads. In auto, SR tends to kick you down to 1st whenever you hit a weird bump or your suspension bottoms out. High will help you keep a steadier speed.

Use auto to get going, then switch to high until you top out, then switch to auto again as you need more speed. Also use it to "engine brake" if you don't want to jostle your entire load when slowing down.

High Gear puts the vehicle in its most energy-efficient state but can only be used on smooth flat roads. In real cars, higher gears allow the wheels to be turned closer to the same rate that the engine is turning. This uses less energy and allows the driver to hit higher speeds. In SnowRunner, High Gear allows the player to save on precious fuel as long as they can maintain a reasonable speed. This is a lifesaver near the end of long trips hauling large materials.

Taken from: https://screenrant.com/snowrunner-mud-snow-gearbox-guide/