See IRS publication 463 in chapter 4.
https://www.irs.gov/forms-pubs/about-publication-463
Generally, assets (property with a useful life of more than one year) must be depreciated, or deducted, over the expected life of the property.
For a car, you choose between 2 methods of deducting your expenses, the standard mileage rate or the actual expense method. With the standard mileage rate, you deduct a certain amount per mile driven for business use (currently 65.5 cents per mile although the IRS updates this figure once or twice a year). You must have a mileage diary, log or app that tracks the date, business purpose, and amount of the mileage. The standard rate includes allowances for fuel, maintenance, repairs, insurance, and depreciation.
With the exact expense method, you must still track your business miles as well as keep track of the total miles driven. You also track all your vehicle expenses (fuel, maintenance, repairs, insurance, and depreciation), and you can deduct a percentage of expenses equal to the percentage of business miles drive. For example, if you drive 10,000 miles for Uber and 6,000 miles for personal use, your business percentage is 62.5%.
If you use the vehicle more than 50% for business, there may be ways of claiming the depreciation faster. But this gets very complicated if your percentage of business use changes in future years, and you may have to pay back some of the depreciation deduction you claimed if your business use changes.
Most people will do better with the standard mileage rate, although it depends on the exact cost of the car and how long you plan to keep using it in business.
