I think adding a MPG meter into the apps tab as a debug tool or implement it into the tachometer. Well.... what do you think?
Already exists. Grab the distance app and click on it to switch modes. One of them is fuel efficiency.
Reset it, it will go to 0 sometimes when your car is stopped (MPG is calculated as Miles/Gallons of fuel used, if you car is stopped you are using fuel but no miles are being travelled, so it will divide 0/fuel used which results in 0. When using l/100km it will just go to a ridiculous high number)
A car is never stopped , because the trip computer even calculates the minute inches that you travel back and forth with the parking brake on.
Even in that case, the tiny movement compared to the normal fuel consumption, even idling, will upset the MPG average at first
The minute inches you don't move back and forth with the parking brake on... Sensor resolution isn't high enough to sense a movement shorter than about an inch