Supernova Cruising RPM Question, and MPG Question

[38Chevy454]

Since I drove my 2008 Supernova 6331 home 1000 miles, I got some good exposure to the rpm and fuel.

I held to 65 mph to try and save a bit of fuel, and the rpm is about 2800 at that speed. Does this mean I have the lower 6.17 (?) gears instead of the 5.29 gears in the rearend? Seems quite high to me with the OD trans, even with the taller 255/70R22.5 tires. My old 88 Ford F-250 4x4 Diesel truck, with the pre-powerstroke 7.3 IDI engine, ZF-5 manual OD trans and 4.10 gears, will be about 2500 rpm at 75 mph. Are there other rearend gear options besodes the 5.29 ratio? Anyone run a different ratio by chance?

Is my rpm indicative of what the lower gears run? Sure seems like it to me. If so I wonder what it would take to see if International would change them out as part of the orig swap they did on some models? Anyone have info on that? Diesels always do better with less rpm and steady state at 2800 rpm can't be helping mpg.

Also mpg related. The dash instant mpg readout would indicate 4-8 mpg most of the time, call it 6 mpg average. However, based on the fuel used and approximating, I think it was closer to 8, maybe 9 mpg actual. I only filled the tanks to top once, so I did not really get a full tank to full tank comparison for a better idea. Is 8-9 mpg about normal range, especially for the lower rearend gears? Why is the dash readout so incorrect? I checked the speedo against my GPS unit and it matches real close, so it is not a problem there.

Thanks for any info, Terry