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The Nat. Semi. part is a linear regulator. A lot of the numbers in their datasheet seem to be assuming Vin-Vout is greater than or equal to 3V, so you'd need 15v in to get 12v out to be within their specs. It might work with less, but I don't know.
As noted before, what is really needed is some kind of switching power supply that can boost the voltage up to 12V if necessary and there isn't a magic bullet solution (i.e. solder a couple of inexpensive parts together) for one of those.
Where I work, we have engineers who design power supplies. Admittedly they're for applications more critical than a car computer, but they specialize in that area because it isn't easy to design good ones.
Alan
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