To set the price of a litre/gallon of fuel you must:

2. Inside the Fuel tab, select the fraction of fuel to consider for the rest of the configuration. The number corresponds to the fraction of the tank, meaning the number 4 corresponds to 1/4 of the tank, and so on

3. Also inside Fuel, type the price of each fuel type. Don't forget to click on SAVE after every modification

4. Go to Cars, and then Models

5. Click on the pencil on the upper-right corner to edit a car

6. Inside the car configuration, choose the fuel type and fill in the tank size and click on SAVE