The rental prices on the website don't include fuel costs .
The renter must therefore return your vehicle with the same amount of fuel as it had at the start of the rental. You are not required to provide the vehicle with a full tank, but this does make it easier to check the gauge at the end. You have to fill at least a quarter of the tank at the beginning of the rental.
If the driver returns the car with a lower level of gas, you can proceed to an adjustment for classic rentals. For Open rentals the fuel adjustment will be done automatically if your car is compatible, find out more about Open adjustments here.