Final drive ratio

Good day, I am an electrical engineer – i.e. in the field of vehicle technology (interested) lay. For the company we need a new transporter with high payload and large trailer load. It should be the new transit FT 350 combi with 103kW. Now one has to choose between two final translations. One translation leads to a lower consumption, the other is recommended for trailer operation. Therefore my questions: 1. What does one understand about final drive or what effects one has to In practice, 2. Does the choice of axle ratio influence the maximum speed? 3. Is there any other reason for this or that axle ratio? 4. The reason for the extra consumption is not clear to me – can you not simply choose another gear and then drive just as sparingly? Many greetings and thanks for the answers. Stephan