MsElectric - I am not a CPA, so seek professional assistance to confirm my statements.
As a Sole Proprietor, I am assuming that you are not a 'C' corp, 'S' corp or LLC and file your business expenses on your Schedule C of the 1040 form?
1. The IRS requires that you keep a detailed log of all mileage driven on a vehicle, both personal and business, in order to determine the actual percentage of business use and mileage driven.
2. The log book (available at OD or Staples) has the Year beginning mileage, Year ending mileage and then you must enter each daily trips information: Begin Mileage, End Mileage and Purpose as well as personal or business
3. Leases DO NOT depreciate, as you do not own the vehicle. You simply write the amount of the lease payment off (plus tax) as a Vehicle Expense. In your case, if your log book determines you used the vehcile at 20% busines, than 20% of the lease payment is a business expense.
4. Any maintenance expenses, tolls, parking, etc. are also deductible. You must journalize all expenses and keep receipts for any expenses over $25. If these are journalized as business related expenses, then they are 100% expensed, not 20%. Tires would be 20% deductible.
5. If you BUY a car, then the car is amortised and depreciated. I won't get into those details, as it is more complicated.
Needless to say, the reason why most businesses LEASE vehicles is because the lease payment is treated as an immediate business expense for that year and there is not amortization or depreciation to worry about.
Hope that helps. Feel free to ask other questions.
Not a CPA but the biggest benefit in business leasing is you get to write off depreciation which is significantly greater than the mileage deduction in most cases.