Can changing tire size improve gas mileage on a BMW X5?

The manufacture of your vehicle designed the tires to give the best ride, handling, mileage, and overall performance. Changing this tire size will effect one or more of these criteria. Improved mileage may come at a cost of lost performance. The engineers spent many hours deciding on the perfect size for this vehicle. My recommendation is to stick with the OEM size tire that the engineers designed for your vehicle.