When I first started driving in the 90s it seemed like the 4 and 5 speed transmission was the best, modern transmission you could get. They were relatively reliable, and had enough gears to make acceleration, and fuel economy good in my Honda CRX.
Today, I can buy a Honda Odyssey with a 9-speed automatic transmission standard, and 10-speed on the high end model.
**What has changed in transmission engineering, materials science, and technology to allow more gears in the gearbox?**
What are the potential downsides? E.g. is the typical 10-speed as reliable as the old standard 5-speed gearbox?
Edit: To be more clear, this is an ENGINEERING, MATERIALS SCIENCE, and TECHNOLOGY question. I understand the benefits of having more gears for ride quality, and fuel efficiency.
In: Engineering
No one has mentioned that a lot the newfangled units are using planetary gear sets.
In the old style, each gear is a cog on a shaft, and that takes up quite a bit of space.
The use of planetary gears in modern units is a space p-saving feature that allows all those extra ratios.
The [ZF 9HP is one of the pioneering examples of this.](https://en.wikipedia.org/wiki/ZF_9HP_transmission)
It’s really complicated but the planetary gear sets perform some sort of voodoo that offers more gear rations in a fraction of the space.
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