Here is a tutorial of how to do it: use MB SD Connect C4 mux to enable tank info & L/100KM into KM/L mercedes W211 (W203) (w221) and more

In DAS Developer mode :
Kombiinstrument -> Diagnose nach Werkangabe -> Steuergerate-Anpassungen ->
Steuergerate-Anpassungen (Variantencoderierung) -> Variantkodierung Werkeinstellungen ->

In the menu that shows , you scroll down untill you read the line : Tankinfo im Reichweitebild
enable this option
now you have tankinfo in the screen that otherwise only showed range

If you allso wanna adjust the way fuel consumtion is showed
you scroll a little more down untill you read the line : Verbrauchanzeige km
you can change it from L/100km into km/l

Scroll down and confirm Coding by pressingF3

if you watch your Instrument Cluster , you notice that scrolling through the menu , nothing changed .

For this , the “Kombiinstrument” needs a reboot.
to do this in Developer mode we go back by hitting F1 -> 3 times
then choose Ansteuerungen -> Gesamstliste aller Ansteuerungen ->
Scroll down untill you see the line : Steuergeraet Resetten (PowerON-Modus) (FN_Reset_SG_Power_ON)
Press F3 and confirm again with F3
Now you will see the “Kombiinstrument” reboots