It takes about 10 km(approx 6.2 mi) of traveling to fill one tank.
You can triple the speed of filling tanks by redeeming your gems using the "3x Boost" button in the upper right corner of the tank or by redeeming gems for "Tank Accelerator."
If tanks don't fill up faster even after trying the above measures
It is possible that location measurements may have temporarily stopped or were not taken while the app was not in use.
If the problem persists, try the procedures described in [I am traveling but the tank is not filling].