Your deductible mileage is the mileage when you are available for service. If you get in your car at home and log into the app, then drive to the downtown area where the bars are (or someplace else where you think you are likely to get fares) then your starting mileage is deductible even if you don't have a fare. If you log off the app after you drop off your last fare, and drive home, that mileage is not deductible if you are not available for hire.
Your available mileage will be different from that shown by the Uber app. You should keep your own separate record. You can use a paper log book/diary made for this purpose, or you can use a smartphone app. Record the date, start time, start odometer, business purpose (driving for Uber), end time, and end odometer, for each day that you drive. If you sign off for part of the day for meals or to do personal errands, note your sign off and sign on times and odometer readings as well so you don't try and deduct personal miles. For most days, you will have one or more Uber fares during the time you are online and available that you can match to your log of availability if you are audited. But your deductible miles are when you are available for a fare.
