To configure the fuel charge for each vehicle class, go to Settings → Vehicle Class. On this page, choose the vehicle class you want to edit, then enter the price amount in the "Fuel Charge per 1/8" field within the Rental Process section.
Once this field is configured, fuel charges will automatically apply whenever there is a fuel difference between the return and pickup steps in a reservation.
Navigate to Settings → Car Rental highlighted in dark blue in the picture below.
Next, navigate to the Default Values section. Here, under Default Fuel Price Per Liter or Gallon, set the amount of your fuel price as shown in the picture below highlighted in red.
