Hello,
Thank you for your question.
There is one setting that can help you out and it is: GNSS Fuel Counter (under Features in configurator- commonly is used if there are no additional accessories to monitor fuel consumption..You can just simply enter right fuel consumption values and get accurate data of fuel consumption under I/O elements> Fuel Used GPS and Fuel Rate GPS.See picture below:
Also to get get accurate values you can check with your vehicle manufacturer whats the consumption values are.
Detailed information about how its being calculated you can find in Teltonika wiki page: https://wiki.teltonika.lt/view/FMB920_Features_settings#GNSS_Fuel_Counter
Hope this will help.
Regards.