Ironic isn't it, that generally the most fuel efficient way to drive a hybrid is to minimize use of the electric motor for both propulsion and regen.
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That's REALLY interesting that Toyota didn't include failsafe protection for the NIMH pack in the event of running out of gas.
Maybe they did it for legal reasons:
Imagine if a Prius ran out of fuel and then got creamed by a truck because the software shut it down when the driver was still trying to press on. Flattened like a pancake even though there was still "plenty" of energy remaining in the battery to drive the car! In some litigious locales, I'm sure there are lawyers who would love to get their claws into a case like that.