Here’s the information I’ve found.
The rearend is a SQHD-R made by Rockwell heavy truck division. These were very common in the 70s and 80s and used by almost all oem.
These are simple to re-gear to get the rpm just right for your engine/ transmission/tire configuration.
Mike, thanks for doing my legwork on rear ends! I'm rolling the dice with mine. I know the 8V92 V8 two stroke turns higher RPMs than the ISM I6 four stroke. The 8V92 was coupled with an Allison HT 740 which has a 1.00 fourth gear ratio. My ZF is currently programmed as a 5 speed. 4th has a 1.00 ratio matching the Allison, and 5th is 0.83, I'm hoping that will offset the RPM difference.
1st - 3.43 2nd - 2.01 3rd - 1.42 4th - 1.00 5th - 0.83 6th - 0.59
If I decide to program it into a 6 speed I'll definitely be looking for different gearing.
I was always under the impression that DD 2 strokes ran at a lower rpm than 4 stroke diesels. 65 mph in ours is 1800 rpm. Red line is 2100.
This is from the Cummins ISM brochure;
"The engine delivers outstanding performance in the 1200-
1800 rpm range. For an optimal balance between performance
and fuel economy gear to cruise in the 1525-1625 RPM range."
Rockwell meritor sqhd rearend is made in several configuration from 2.82 to 6 plus. Since the coach was produced when 55 was the norm, changing to today’s faster speeds can be a challenge. First find the recommend sweet spot from the engine Mfg for engine rpm then determine your speed you want to travel at that rpm. Then match the tires and the rearend ratio to make it happen. The engine rpm is the priority due to elevated temperatures are our worst fears. Also install a pyrometer and boost gauge to confirm temperatures. IMHO
https://www.dieselhub.com/tech/ratio-tool.html
Use this handy tool to make comparisons.
Good news. Finally the rains have stopped and it has dried out enough that I was able to Jack the passenger side rear tire and spin the tire one revolution and count the drive shaft revolutions and it seems that I in fact do have a 2.82 rear end ratio differential. (Mike Ellis was correct) Now to set the dip switches for the correct rpm of about 1400 @ 60mph. Thanks for all the encouragement and advice.
Now we can get down to planning our trip soon. Any ideas in Texas?? Any one want to plan a Newell get together in the Texas five state area?
I have an 83/6v92TA I also am going through the regearing question. My tach was funky but semed to be reading 2k at 70. My mechanic thought he may be able to make a 2 speed rear end work in there.... but if my calcs are right a 2.82 would give me 1700-1800 at 75 which would be perfect for my highway driving.