Yes, contemporaneous records are given "great deference" by the IRS, so start NOW to note all business trips in a notebook in your vehicle. Easiest way is to enter date, starting odometer and ending. Then if you have several days of no business trips, enter something like, "Aug 1-4, errands," and the end mileage for that. Then "Aug 5, address of shop, end mileage." This way, each end mileage note is clearly also the beginning mileage for your next trip or series of trips, in the case of several non-business days. Use the computer generated mileage ONLY for the months when you have no actual records, BUT you must report the odometer reading for January 1, 2018 and end of Dec. 31, 2018 in order to compute percent of total miles used for business on supporting IRS forms that feed Schedule C.
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.