When performing a chip tuning, the MFA (Multi-Function Display) should display the fuel consumption "just as accurately" as before the tuning, because the electronics can "understand" how much fuel is being injected.
In the process of box tuning, the electronics are programmed to simulate a higher actual dosage (e.g., it thinks there are 40 mg per work cycle, while it is actually 50 mg). Therefore, a lower-than-actual fuel consumption is displayed.
The current consumption is calculated based on the injection volume per working stroke, the engine speed, and the speed. The average is calculated from the accumulated volume and the distance traveled since the last reset.
However, there is also the possibility to adjust the MFA display to match {REQUIREMENT}.
to adjust the actual fuel consumption. However, this requires (as is unfortunately the case almost always) a diagnostic system.