Trip Fuel Cost
From Torque Wiki
⧼monobook-jumptonavigation⧽⧼monobook-jumptosearch⧽Trip fuel cost shows (in the units you have selected) the total cost of fuel used for the trip.
This display depends on the Vehicle profile being filled out with accurate information. Fuel type and Displacement are important items to fill out for this display. Make sure you have told Torque to use the vehicle profile after filling out the information.
More information on setting up for MPG/Fuel efficiency can be found here