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The problem is that while hydraulic has a good cycle efficiency - that is, you get back a lot of the braking energy - there's not that much stored energy. The storage tends to be heavy, too, because it's stored by compressing gas to high pressure. That's why you see the hydraulic hybrids used in UPS delivery vans, garbage trucks, and other places with a frequent start-stop cycle.
For a car to be practical (around here, anyway), you need a powerplant plus storage that can power it up 5000 vertical feet of 7% grade at highway speed.
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