I have heard some rules of thumb, putting it around 5-10 mph at top speed, but no scientific calcs.Makes perfect sense to me and adding water down low at 8.3 pounds to the gallon will definitely make a difference! Have you done any number crunching to see if the larger tires will improve your road speed? I know they won't do a lot when climbing a long grade, but on flat ground you might gain a few miles an hour maybe?
Again... A cool truck and with your extra features it will be even better!
Yea I am looking forward to this being a long running project. I got a lot of ideas, just pulling them off will be the true test, ha!