I was reading some posts a while back about some folks getting good mileage out of their 7.3L and some not so. It appeared to be the same truck for the most part but some where getting in the high teens or better. I get about 17mpg's with my 99 F350 7.3L.
Mine is a early 99 model. Here is a video explaining an upgrade that can be done to increase my engines power. In short: the later 99 models and up to the 2003's have a different set of injectors which deliver about 30% more fuel to the cylinder. Increased power? absolutely- less mileage - more than likely.
I have no intentions of changing my injectors to the larger units as my mileage would drop, I am certain. And I am quite happy with the power my truck has, as is.
I know some folks don't like this guy, but it made sense to me. I may do the electronics upgrade some day though
Here it is