Probably about 1-3K rpm. They usually turn about 2c the engine speed and
won't provide enough power for a lighted car at idle although they are
close. Consider idle speed to be about 500 rpm.
For a single car headlight you may be able to get by wit a bit less speed.
Probably the best thing is to hook it up to a drill press or lathe and see
what works.

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Why isn't there an Ozone Hole at the NORTH Pole?