Your data
Export and back up
Settings → Data → Your data → Export everything (CSV) is the backup, free at every level.
Everything
Every vehicle with its settings, every entry as stored, and your app settings (region, units, currency, home and business rates, display). Import it with Import a CSV, on this phone or a new one, to restore it.
One log at a time
Or one log at a time, for the vehicles shown: Fuel log, Charging, Mileage log, Maintenance, Expenses and Gig pay (CSV) there, and the business mileage in Settings → Prices → Business mileage. They open in Excel or Google Sheets.
Their units
Those logs are in your units (now miles, gallons and MPG; Settings → Region → Units), with the unit in every column’s name.
A car whose odometer has its own unit (a rental from Canada reading km) has its odometers in it in the Mileage log, an Odometer Unit column saying which; the other logs keep one unit a column, yours.
The backup keeps what’s stored, miles and US gallons, so it restores the same whatever the units. A rental’s dates and odometer unit go with it, and a borrowed car’s, whose it is and what was agreed (its condition photos stay on the phone, counted, as every entry’s photos are).
Restored, a rental only goes onto a rental and a borrowed car onto a borrowed car. Importing another app’s file never makes either.
What isn’t in them
Photos, receipts and scans aren’t in the files: keep them on the phone, or save them yourself.