Hello,
Thank you for your question.
GNSS Fuel Counter - commonly is used to 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 can be found in Teltonika wiki page: https://wiki.teltonika.lt/view/FMB130_Features_settings#GNSS_Fuel_Counter
Hope this will help. If no let me know.
Regards.