To prove you are eligible to deduct your car and truck expenses, you should keep a mileage log. A mileage log needs to contain the date of each tax-deductible trip you make, showing how many miles you drove and for what purpose. You will also need to know the total number of miles you drove for the year, so it would be a good idea to indicate your odometer reading at the first day and last day of each year. This mileage log should be updated as a trip is made, not created later. Record your mileage at the start of the trip and at the end.
All records must support the amount, time and place, business purpose, and business relationship. Be as specific as you can. Entries that are too general can be disallowed. Remember that IRS audits usually take place a year or two more after the event and record-keeping is key to your successful outcome.