Full-to-full. When you fill the tank, Full Tank takes the miles driven since the previous full tank and divides them by the gallons added since then. Your first fill-up has no MPG because there is nothing to compare it with.

Turn off Filled to full on the entry. It is saved without an MPG. Its gallons are added to the next full fill-up, and MPG is calculated over the whole span.

Save a backup first in More, Export and backup. Then open More, Vehicles, choose the vehicle and tap Delete vehicle and all its records.

Open More, Import from file and choose a CSV you exported from your old app. If columns are not recognized, match them yourself and choose the units. Full Tank shows an MPG check before anything is imported. The Move from Fuelly page has the steps for Fuelly.

In More, Export and backup, tap Save a backup. To restore, open More, Import from file and pick the backup file. Nothing already on the device is replaced.

iCloud sync is part of Pro. Turn it on in More, Settings, Sync with iCloud. The change applies the next time you open the app. Your records sync through your private iCloud account, so you need to be signed in to iCloud on each device.

Purchases are handled by Apple, so refunds go through Apple. Request one at reportaproblem.apple.com. Restore purchases in More, Settings brings Pro back on a new device.