It's not done according to the Financial Accounting Standards Board; but for about 30 years, I always used one car dedicated for Real Estate, and I took the mileage on January 1st each year and subtracted it from the recorded mileage on the following December 31st to obtain an annual figure.
From this annual figure, I then subtracted all known personal trips, and attributed all of the remainder to business usage, and I applied the IRS Mileage Rate for a deduction, and was never audited (for this).
Here in tiny little Vermont, I sometimes drove over 44,000 miles per year . . . . sometimes like a pin ball machine, and well over 1,000,000 miles; but I spent the majority of my time on the actual Real Estate Business and NOT on filling out non-productive Logs to keep track of every single one of my dinky business trips.
That's just how I did it; but it's not for everyone . . . . talk to your Accountant or Tax Preparer about what will work for you. Record keeping (for this and other matters) can be a real pain; and you don't want to get stuck changing techniques midway through a Tax Year and have to construct a revised accounting. I'm my own Accountant and I prepare my own Tax Filings.
I don't know if there's an IRS Approved App that can be tied into your GPS and produces anything that would be an acceptable written record at Audit Time . . . . maybe one is coming down the Pike ?
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Dale C. Hittle of GOLDEN RULE PROPERTIES in Glover, Vermont
Where We're Always Striving To Put Together "THE FAIR DEAL"