What is the difference in a 21 speed to an 18 speed in driving semi?

The difference between a 21-speed and an 18-speed transmission in a semi-truck lies in the number of gears available for different driving situations. Having more gears in a 21-speed allows for finer control over the engine's RPM and speed, providing better efficiency and performance compared to an 18-speed transmission. This can be especially beneficial in varying terrain or road conditions.