Automatic Calculation
 

In the free/old version of the app and on the web, the number of liters is automatically calculated when I enter the total cost and the cost by liter, which are the two easiest numbers to remember between the fuel-up and the data entry (in fact, it can calculate the missing information after you enter 2 out of 3).

But the new version of the iOS app doesn't do that. We have to enter all 3 data points. Am I missing something obvious? Of is the app really missing this important feature?

The app should be calculating automatically when entering two of the three data points. If you continue to have trouble, verify that your data has synced to your Fuelly profile, if yes then delete the app and re-install from the app store. Login from Gas Cubby by clicking "Set Up Sync" and the data will sync back to your phone from Fuelly.