If your roof rack is higher in bulk, then it can affect gas mileage badly. Otherwise, it’s all cool to drive with that. The reason why it causes fuel consumption is that the truck drives imbalance if carrying big loads. Apart from that, it can result in the truck driving with good speed while holding loads of burden on top of the roof. By that, the truck has to use more fuel to drive straight and in good shape. FYI, the unfilled roof rack can impact the truck fuel system around 2% that is 1/4 of a gallon of gas.