On our 9,000 mile trip this summer, the trip average in my 1999 Thomas bus with 3126b CAT (electronic engine) and 6-speed Allison MD3060 (6th gear enabled), cruising at 65 with foot buried most of the trip, weighing in at about 24,500 lbs, we averaged 8.2 mpg. Best was 9.3 going SE through Yellowstone on 287, worst was on PCH (route 1) at about 6.5 (constant hills and stop-and-go letting people pass).
Edit: 4.62 rear gears turning ~2,100 rpm at 65.
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