How do you shift tractor trailer gears?

If you have a low speed such as an 8-10 it is a simple double clutch and shift. But, if you have 13-18 you will have a low and high range switch. 18 Speed is simple for me

It is in low range shift 1-4 at 4 switch to high range. Shift 4-8. So, in reality you are not actually shifting 18 times.