Expenses for an automobile used in business can be quite valuable at tax time. However, your business need not own the car in order to deduct mileage and other costs. In fact, you can deduct just as much if the vehicle remains in your name. It is also less complicated than titling the car to the business, particularly if you have partners. The simplest way to deduct costs against your business income is with an “accountable plan”, whereby you submit your costs, mileage, etc. to the business, which reimburses you.
By the way, sticking an advertisement on the door of your car does not mean the whole car is deductible. The sticker is deductible, but the portion of the car that is deductible is still based upon the proportion of business to personal miles driven.