I think the fuel economy will remain where it is. I'm surprised that you say you're not impressed with the FE in your '03. Granted, it's a dually and you have a cap. Both draw FE down. But what do you get, hand calculated?
Hand calculated my last tank was 9. 2 mpg. Being winter and all the idle time and crappy fuel,I don't expect to get very good mileage. The best mileage I've ever gotten was 18. 5 mpg on a road trip to Fairbanks and that's using my Smarty and driving at 55 mpg at whole way. I've never seen 18 mpg since. I don't do any city driving to speak of since we live out in the country, its mostly 20 mile- round trips into town at 55-60 mpg. I usually see between 14-15 mpg. The Smarty made the difference for me, was lucky to get close to 15 mpg on road trips. I always write down any changes in my fuel additives to see what works best. My mileage suffers if I don't use Power Service/two stroke oil in my fuel. Its rare if I drive my truck hard,I always try to get the best mpg each tank,except in winter. If we lived in the lower 48 and had nice highways and didn't have to deal with hills and crappy roads,it might make a difference. I'm waiting on our tax return so I can have my trans rebuilt,I'm curious to see if it makes any difference.
I'd like to have my cake and eat it too,as far as torque,power and mileage goes.