If I'm not mistaken, the output through the transfer case is about 1.4 to 1, so it would be higher without the transfer case. My advice would be to run the stock size tires, 205 70 15, I think, and it should get 20+ mpg. You might look for ways to lighten it up: remove the rear seat, use a ragtop, even remove the front driveshaft. The dummy tcase sound like a good idea, if you could find one, but the front gears would still be there, so you would need the front axle from a 2wd as well.