Color me confused, but why not do a 2 stage regulation? The mobile vehicle power system is horrible, to put it mildly. I can't speak for bikes, but voltage swings from just under 12V (before start) to 15+ are common in cars. As well as all manner of hash, noise,etc. If you still want to use the arduino regulator, stick a L7809ABV (SMTMicro) between the car's supply, and the arduino. That will take 10.5-26V, and convert to regulated 9V - which you can then feed the arduino. Yes, it's a little lossy, but it provides a secondary level of protection. (This assumes you don't need 12V) the LM7812ACT would give you 12V, but it needs 14V in - a bit iffy when the engine is not running.