Can mileage be doubled to 20-23mpg? Which programmers are reliable and for real?

I would leave well enough alone. A friend of mine changed the chip on his 7.3L PSD and it made very little difference, except in his wallet.

The Texas greasmonkey has it right! Leave it alone. I put a BullyDog chip in my '04. Wow, it really boosted the speed and power but I do believe the turbo died from it. I put it back stock. Good thing it was warranted. You might as well put cow magnets add the little turbo air thingy, put a water injector on and all the rest of the BS that is out there. Nothing will increase your fuel milage. Good luck TS