Every time you get gas write your odometer reading on the receipt. I would recommend doing it as soon as you get back in the car from pumping the gas. The mileage progression is recorded throughout the year with this method. In an IRS audit the auditor wants to see a repair bill with the odometer reading sometime at the beginning of the year and then around the end of the year or shortly thereafter. With your written notes on the gas receipts further support is added toward the amount of mileage you drove throughout the year. It also provides more credibility to the auditor that you are being diligent about maintaining records.