The problem is that if you use the actual expense method, you must know all the expenses for the year, and all the mileage for the year, and your business miles, and allocate expenses on a percentage basis. For example, if you pay for an oil change, you can't deduct the entire cost as a business expense unless 100% of vehicle miles are for your business. If only 50% of miles were for the business, you can only deduct 50% of the cost of the oil change. Same thing with a tank of gas. If you can get all the expense records, and you know all the personal and business miles, you can make it work. And, you can't include depreciation, which is one of the allowable expenses, because you are not the owner.
More details in publication 463, chapter 4.
https://www.irs.gov/publications/p463
