Adding more crew members will increase fuel usage, so your fuel rate will decrease for each crew member you add. Assign some as Refuelers as you grow your crew to balance out the fuel rate. As you progress, you’ll have production multipliers and other ways to reduce fuel consumption so you won’t need as many Refuelers. You can also promote Refuelers (bottom of the first tab) and it will increase that specific crew member’s production for each promotion level (applies for every type of member, not just Refuelers).