Yes. At the time of this writing, IRS Form 2106 instructions (for 2008 returns) had this to say about record keeping for your vehicle: “You cannot deduct expenses for travel…unless you keep records to prove the time, place, (and) business purpose…of these expenses.”
I keep a small spiral notebook in the visor of my truck. Each morning I list the names of the jobs I’ll be doing and the current odometer reading. As I go from job to job, I record the odometer reading. At the end of the month, this page is torn from the book and added to the file that I keep for each month’s receipts. (Every month has its own folder.) When matched with invoices these logs serve as my written record of mileage and business purpose.
In the same spiral notebook; record the money you spend for gas, insurance, registration, maintenance and repairs of the vehicle. Save your receipts. At year end you will decide whether to take the standard mileage rate or actual expenses on your taxes. Check the tax codes for limitations and for the current mileage deduction rates.