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Originally Posted by MGBGT
It's interesting how the technical pundits at Daimler Chrysler, GM, and other places can come up with all these technical reasons why HSD is suboptimal.
The bottom line is though that the Prius II gets better mileage at any speed, than any of their gasoline cars, period.
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GM has not actually implemented their hybrid system on any vehicels yet, and will not do so it seems until 2008. Based on the description, the transmission does seem to be somewhat better suited for fast highway cruising:
http://en.wikipedia.org/wiki/Advanced_Hybrid_System_2
one thing I immediately noticed is that unlike the Toyota system with a single planetary gearset, the GM/DC system has two planetary gearsets. This allows for both the torque-blending CVT technique of the prius, and the ability to actually switch from a "low" to a "high" gear. The problem with the Toyota transmission is that it is essentially a fixed-gear, that must adjust the appropriate level of torque using generators and electric motors. This wastes a lot of energy at fast steady speeds. GM/DC's system does this as well, but with the addition of a true high gear for the highway. In "low" mode, it's essentially the same as the Toyota HSD transmission.