Torque can use several sensors to calculate your vehicle MPG / Fuel efficiency
Due to the different sensors some vehicles use, the calculation may require information to be entered into the vehicle profile to be accurate in some cases.
There is also a newer 'alternate' MPG calculation that can be enabled in the OBD2 Adapter Settings, to enable it, tick the box labelled 'Enhanced MPG calculation'.
The Trip Average MPG is calculated from distance travelled / fuel consumed by the vehicle.
The 'Long Term Average' MPG is remembered and is stored as part of the vehicle profile.