This option is very useful. With manual gear changes, you can use the Q and E keys to change gears. Change down gears with Q. You can even switch to reverse gear. That would be very interesting to use in Arma IV for “much immersion.” It is apparent that changing down gears when going uphill gives you more speed. It is a very good feature. I am not sure if you can switch to a certain gear, but this still is great to use.
I do not think the engine is damaged by over-revving it, but it should be. With the manual vehicle control in Arma Reforger, you will also be required to hold R to start the engine. The ability to change the Field Of View in the game allows a better view of objects in the distance.