Fuelly.com and our apps will allow you to enter "0" (zero) under price per unit and total costs if you choose not to track that field.
However, you can also simplify your fuel up screen by toggling off the Fuel Costs field.
Please refer to FAQ:
FAQ: Visible Fields: What is it and what does it do? for more information on how you can toggle Fuel Costs and other fields on/off from your New Fuel-Up screen.
To view all our FAQ's:
Frequently Asked Questions | Fuelly