im familiar with the lm317, last year i built a variable voltage power supply in my electronics class. its actually what im using to test alot of these circuits.
i know the voltage in the vehicles will vary from something like 11 to a max of 14.5. the low end doesnt matter to me, because it wont hurt. but the spike could. i am ultimately designing these to be durable. i think i will experiment with the resistor and see if it performs like i want.
thanks for your help guys.