If you read the article is says that dual clutch automatics are much faster than manual transmission and most modern automatics have 7 to 10 gears for better fuel efficiency but no one is willing to use a manual transmission with that many gears.
I believe every US market Nissan sedan, including the stanza at the lowest end, come standard with a continuously variable transmission programmed to fake the sensation of shifting at fixed gear intervals. so in that case, you get an infinity of 'gears'
I'd take the manufacturer's efficiency claims with a grain of salt; they are not going to be individually reproducible, let alone comparable to each other for those models. You can drive a manual for fuel efficiency, and be assiduous in doing so; there is intent and skill involved.
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.
I'm sure they would love one, but it's probably not that easy/cheap to find one that can handle that much torque with such heavy loads. The biggest/heaviest trucks just end up using electric/traction motors running from diesel generators, so they don't need a transmission at all.
