aCar can switch between used units at any time and make conversion of stored data. Go to app settings and change fuel unit, confirm re-calculations and make a standard backup :)
aCar in free version makes monthly backups to your phone memory. These are located at:
Phone memory / Zonewalker-acar /aCar-auto-XXXXXX-XXXX.abp
It is also possible to trick application to make such backup without waiting full month. You may change date and time on your phone!