HD, the only was you are going to see significant MPG gains from SeaFoam is if your intake & injectors were in desperate need of a good cleaning. The most noticeable improvement is usually throttle response &/or a smoother idle. In addition, a change in tire size won't necessarily make a huge difference either. Remember, these trucks came from the factory as gas hogs, there is little you can do to change that. Just keep it maintained so you can maximumize the efficiency potential.
The biggest MPG increase I ever had was when I changed my spark plugs (Champion copper core gapped at 0.040), plug wires (MSD 8.8mm), and new distributor cap (brass contacts) & rotor. Of course, only the 5.x engines have wiresets and caps/rotors
, so that doesn't matter on the other motor. You get the point though.

 

Hey Folks, 1 month update!

Ran 1/3 can into the vacuum lines 2/3 into the fuel tank. Improved the response noticeably. Waited a week and ran a second can, 2/3 intake and 1/3 fuel, changed out the plugs and swapped the 16 inch tires for a set of 15 inch tires and rims off a 2000 2wd parts truck. Now about 3 weeks the 'kota is running pretty good. Response is much improved, power+, shifting points are closer to where I think they should be and here's the kicker mileage went from 13-14 to 17. !! (I hate 'story' problems, math !@#$) The 'kota is setting alittle lower but I haven't hung it up yet on the trails. The Seafoam and the tires made a huge difference in how the truck was running, and for 3 mpg I'll give up the bigger tires for now.