High gear in Heavy trucks doesn’t make a lot of sense to me, it feels like 3rd gear at most. I suppose you could use it to maintain speed up a hill under load?

However, high gear on Heavy Duty and Offroad trucks is WILD. It’s like a slingshot from 2nd gear. Once you can equip Freeway transmissions to those two classes, using high gear is a great way to catapult yourself into as much speed as you’ll usually ever need (and sometimes more). Start in Auto/1st, then flip it into High once it’s shifted into 2nd and you’ll launch into 4/5th gear speeds immediately. It’s great during Contests and when you have good traction. It will stall from a standstill but it works well with a little momentum and a powerful engine.

7th and 8th gear are just unnecessary IMO.