Will the arduino be running while the auto is off, or just while engine is running?While running voltage will get as high as 14.8volts. How many current hungry devices will be drawing from the arduino (relays, etc)?14.8 volts is a bit high to send into Vin with any current usage. If you are not using much current it may be OK. Otherwise, I would put a voltage regulator to drop the 14.8 down to about 7 volts, to feed to the arduino. Makes for a much happier baby.What boards are you driving from the arduino?good luckJack
dfrobot which you already found have suitable power supplies too and the price of that seems reasonable.
have a look at this - http://ruggedcircuits.com/html/ruggeduino.html -
But I can't see heat sink on this converter?