Trucks don't have 18 gears for efficiency. They have 18 gears because low-revving diesel engines have small power bands; mix that with heavy loads, and it pretty much necessitates having a lot of short, closely-spaced gear ratios, so that maximum torque can be achieved consistently, no matter what speed you're at.

Also, FWIW, they don't just pick between 18 gears with a single shifter. They have multiple inputs that allow shifting between multiple combinations of gears, some requiring clutch to change and some not, as they gain speed. Most trucks still only have 6-ish positions for the main gear shifter to slot in to.