The auto industry is just catching up to the mountain bike industry, that's all
My first bike was a ten speed. Later, my first mountain bike had 18 gears. Later on, 21. A few years ago, 24 was the norm. Now they're at 27, and 30 is coming soon. I wonder if they'll add a fourth crank gear to bump it up to 40
There is more involved in fuel economy increases than the final drive ratio. Some cars get better mileage at 75 mph than at 60 mph, for example, even in the same gear. This is because the motor's maximum efficiency is reached at a particular RPM range, along with a drag coefficient which is low enough that the 15 mph increase isn't defeating the engine's efficiency gains. (That doesn't necessarily mean that only sleek cars meet this description though) It's not
generally "lower RPM = more efficient".
For highway mileage, it's all about the final gear matching up the current travel speed with the engine's efficient RPM range. A lower OD gear isn't always better, if they already have the ideal OD gear. Having more gears in the tranny can help ensure the final gear is
the ideally efficient gear for typical highway speeds, rather than a compromised range, as well as keeping the engine nearer to it's most efficient RPM range while in the varying speeds of city driving.
So, how much mileage increase can be had from a new tranny largely depends on how poorly matched the current tranny's gears are to the engine's max efficiency range.