its simple, u already know that not pressing the pedal it goes to 0, thats because the car is not using any petrol when u dont press the pedal while it is moving (like coasting down a hill). so when you press, it goes full, but it usually wont go full for long unless you depress it for a long period of time. try cruising at 100-120km/h lightfooted, then u will see it will go somewhere between 7-12L somewhere around there..when your car is in a complete stop, it uses maximum petrol..thats why when u're in stop-and-go (aka traffic jam) your car uses a lot more petrol then when it is driving to its destination. so to use or see the instance FC bar, just drive normally and observe the bar go up and down as you press your accelerator pedal. usually the first 4 gears would yield high FC but once u reach fifth and sixth gear you will see the FC getting better.