They do not have any hubs actually. The wheel hub assembly is splined and the axle slides through it and is secured by a large nut. Meaning they work well and are trouble free. The only downfall is the axles turn all of the time regardless if it is in 4x4 or not. The outer axle u-joints fail more often because of this. As for fuel mileage, im sure it hurts somewhat but isnt enough to warrant putting a $3K kit to help out IMHO.