Just my personal experience but I have run a couple trucks hubs locked all winter long and the mileage drop was very small, about a half a mile to the gallon.
In my full time blazer it got 10 mpg before I converted the 203 and it got 10 mpg after I converted it to part time and put hubs on it.
Same thing with a buddys Dodge pickup, he was told expect 2 -3 mpg better fuel mileage and realized less than 1 after we converted his truck.
All these trucks had good running motors and the rest of the drivetrain in good shape