Wonderful and very informative thread! I just finished reading from the start to end! Just want to understand how a tuning box manages to improve the fuel efficiency. 1. The working as i understand A basic tuning box introduces a resistor between the rail pressure sensor and the ECU which modifies the signals and makes the ECU believe that the rail pressure is lower than it should be. (same logic with boost pressure sensor in case of a dual channel box, right?) This makes the ECU pump in more fuel to the rail to increase the rail pressure. So, for the same injection duration, the injectors inject more fuel -> and eventually engine produces more power! Since the injected quantity is increased, where is the scope for FE improvement, it should logically worsen. 2. The way the MID perceives Fuel Consumption: I believe, a car with a tuning box will indicate a lower fuel consumption on the MID. Since the ECU is not aware of the increased rail pressure, its calculation of injected quantity vs energizing time of the injectors will be incorrect (lower) against the actual. (The injectors are injecting more fuel than the ECU thinks due to higher 'actual' rail pressures) Are we being made to believe that the lower consumption shown in the MID is indeed real and it is because of the tuning box? Or is my understanding wrong and am I missing something? Can the fuel consumption value shown on the MID still be trusted?