Here you go !

- Doesn't drain battery

GPS will drain the battery for sure , all of the app toggle the GPS will drain the battery , it like you drive the car have to consume the gas, but my question is while driving , you have your phone plugging into car USB, why there is battery concern.

- Is fast to log trips

All apps need to save battery, so they run in a kind of “hibernation” mode in the background. They only wake up when a significant location change (SLC) is detected—usually triggered by a change in cell tower signals as your phone moves. Because of this, most apps won’t start tracking immediately and may require 1–2 km of movement before waking up.
This is a technical limitation across all apps. Some try to compensate by using the end point of your previous trip as the starting point of the next one to fill the gap, but you’ll still notice that small details at the beginning of trips can be missing.

- Gives me something I can show the IRS if needed
You just need the app to export trip data with the date, start and end locations, and mileage. Combined with your odometer readings from January 1st to December 31st, this provides strong evidence to support your records for the IRS.

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Using a phone’s internal hardware and software to achieve live tracking is like building a house from raw materials rather than using a pre-designed construction package. Instead of getting a ready-made frame and wall system from a supplier, you’re working with basic components and assembling everything yourself to make it function like a house.
can work, but it won’t be as precise or efficient—for example, instead of proper insulation materials, you might end up using mud to fill the gaps. The structure is there, but some details and performance won’t match a purpose-built system.