WOW, your van operates 2 gears LOWER than mine! Going 75 MPH on flat roads I was complaining that it prefers 7th gear (1971 RPM) over eighth gear (1633 RPM). But you are reporting 5th (2815 RPM) and 6th (2280 RPM) gears at that speed. That is really going to take a toll on your engine life turning those RPMs all the time.Excellent spreadsheet, thank you!
The dark window box is the area that I'm typically operating in, and my experience is that the transmission does not like to stay in 6th gear, and the 500-530 rpm difference between 5th and 6th hurts the gas mileage. Getting it to drop down into 6th is hard, and it will pop up into 5th if you breathe wrong.
Engine & tranny seem to really settle in a 65-66 mph = 2450 rpm / 5th gear, which holds firm even on a slight grade. Produces best 29+ MPG vs wind resistance vs grade conditions.
Eighth gear is hard to find when going over 61 MPH, 61 and under it shifts into eighth gear with ease. All this may be due to the way the tranny "learned" my driving habits. YMMV