It looks like 9-speeds (or maybe 10) will be the upper limit for automatic transmissions.
When I was studying engineering back in the late 1980s, three-speed automatic transmissions were still the dominant configuration and 4-speed units were just starting to hit mainstream vehicles. By the late 90s, 5 and 6-speed units were starting to arrive. Today 6-speed units are dominant but German transmission manufacturer ZF is pushing hard with its 8-speed automatic which was recently to several Chrysler models.
Last year at the Detroit Auto Show, ZF and Chrysler announced the development of a new 9-speed automatic for front wheel drive applications and Hyundai announced work on a 10-speed.
The problem is that every time you add more gears, it increases the complexity and weight of the hardware. While more gears allow the engineers to keep the engine running closer to its sweet spot for efficiency, power or torque (unfortunately it's usually not the same spot), as you approach 100% efficiency (never actually achievable) the incremental gains gets smaller while the costs get larger.
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