I pretty sure, whoever is telling you this, has not clue about taxes. Don't do it. Just get some help from qualified tax advisor, you will save much more.
I don't own a part car within my LLC, but I use my personal car for my LLC and S-Corp's business all the time, and I write off mileage for each. It's usually a pretty good write off since I log something like 12,000 miles a year for my business.