Woo, more results. Took me awhile, but, not gonna quick-burn gas for the sake of faster results.

Did more testing, additives, OD on, OD off, swapping around speeds at where I engage/disengage OD, etc. I figured out, the fuel savings of turning OD off at <50 is just about the same as the fuel cost of leaving it on. Fuel additives didn't seem to really change much, as expected, but it never hurts to clean the system a bit. Adjusted the speeds at which I engage OD just a hair, if I can keep a steady 45mph, on good flats, I'll engage OD. My transmission goes into TC lockup at 45 with light throttle. If I can't keep 45 due to hills, or traffic, I turn OD off to keep wear of the lockup solenoid/clutch (since it'll basically act like it's 'hunting' locking, unlocking repeatedly). It also causes the TC to lockup in 3rd, giving me effectively the same as 4th gear in a manual, and cuts down on heat. Overall keeping to the standards I laid out in my opening post, I got a very level mileage. Adjusting my speeds, I managed 19.4mpg. Until I decided to see what happens when I roll down the windows instead of A/C... What happened? My mileage dropped. All the way down to 17.3mpg, might just be an oddity, but I just happened to notice it. If I come up with anything further, I'll post it here.

 

Finally got to take the truck on a purely highway trip... 1077 miles one-way (I ran out of washer fluid... a thousand miles of bugs... every single gas station didn't have washer), have to do it again back later. Doing 75mph all the way, I averaged 21mpg, highest 21.8. I guess compared to what some people state they get with these trucks, I can't complain too much.