Here are the problems with using an app for tracking your miles.
1. You have to make sure the app is started as you begin driving, and then exit the app when you're finished driving, expecially since many GPS apps drain the battery like crazy.
2. You might forget to turn on the app. That will cost you in lost expense reimbursement or tax deduction.
3. Many people upgrade their phone once every year or two. Transferring the data can be a hassle.
There is a better, non-app solution, but unfortunately for you I can't mention it because of the rabid moderators here on TR. Just google something like "gps car mile logger".
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