I wasn't looking for MPG gain, I had a need to replace my bad wheel bearings (Unit Bearings) and have never liked them. The cost to replace bad unit bearings with the same are less than the Free spinning hubs, but for a little more you get real servicable bearings. Another advantage is backing up at 23K GCW up an incline with a manual transmission, or even moving short distance forward, just set them in the free position and 4Lo. Increase in MPG is welcomed, but not the reason I converted.