I have 285 75 17 Toyo A/T. open country. The dealer changed the ABS setting to the 285 70 17 setiing form the power wagon option when i changed tires. the result was that my speedo was 5mph off. When i was at 65mph by my speedo, i was really going 60mph by my gps. Confirmed by 2 different gps devices.
Now that i have Jr i want to recalibrate my ABS setting. I found that setting the tire ratio at 30:20 gives the most accurate ,in terms of speedo and gps in sync. yet the tires measure almost 32.25" tall. go figure?
Anyway, hope it helps anyone who needs to recalibrate with those size tires. I spent 45 mins and lots of fuel on trial and error runs yesterday.

 

in measuring those tires and by their cataloge those tires measure 33.90 tall

 

I know, mine have some wear and measure out less than full tread with 20k miles on them. After 7 runs of recalibrating, with 2 gps devices , that ratio of 30:20 seemed to be dead on when programmed into the Jr.
it doesn't make sence to me either!