Engine RPM - Allison Trans -Rear end gears.. HELP

OK so it seems I have bought the wrong truck. Hindsight is such a wonderful thing. I simply cannot afford to get a different truck at this point so I simply MUST find a way to make this one work. I have done quite a bit of homework and investigating but am running out of new/usable information so I am putting this out to the community for help.

2007 Hino 338.
Allison series 3000 auto trans - 5 speed.
5.57 rear gears.
275-70/22.5 Michelin XZA2 (38" tall)

Factory dash speedometer is off by 6mph so all stated speeds given here are from 2 dash mounted GPS units that have always been in agreement on speed.

The "Green zone" on the tach ends at 2200rpm. Redline starts at 2600rpm (Note: The below testing was done empty. 70mph was reached only long enough to get the RPM reading and then immediately backed off down to below redline)

70mph = 3000rpm
65mph = 2560rpm
55mph = 2150rpm
50mph = 1850rpm

I want (NEED) to be able to go 60 and stay in the 1700-1900rpm range. I am not interested in going any faster than 65 - 70mph max and want / NEED to cruise at 60. Rolling at 50mph on the interstate is not realistic but 55 is tops and at that RPM the fuel mileage is not tenable.

The Allison trans is currently programmed as a 5 speed. I have confirmed that the transmission is 100% compatible with a software update that will allow it to be a 6 speed. All it takes is a code available from Allison and about $200 in shop labor fees. In order to get this code from Allison I need a letter from the chassis manufacturer saying it is ok with them to do this. I just spent 2 days on the phone with Hino factory representatives. I went all the way up as far as to talk to one of the top 5 guys in charge of American Hino operations. The final answer from them is,"Sorry, but no letter". I don't want to discuss getting a letter or what to tell Hino in order to get them to give me a letter. It just is not going to happen. Further discussion about getting a letter is a waste of time at this point.

What I do want to talk about is how to drop the RPM WITHOUT loosing what little power I currently have. The truck has "acceptable" pulling power as it is but the cruise speed RPM is to high. I could change rear gears. I know 4.11 is available for this rear end but I am very concerned it will make the power no longer acceptable. Same goes for larger tires.

Questions:
1) Does anybody know of a shop in the US that is able to reprogram the trans? This is really THE proper answer to this issue and what should have been done from the factory. This would be BY FAR my preference to do.

2) Several years ago (About 1995) I had a 4x4 F350. I had purchased and installed an auxiliary transmission that was installed between the transmission and the transfer case. You could get it configured either as an over or under drive unit. It worked really well. I contacted that company to see if they made a unit for a business class (class 7) truck. Apparently they no longer make those units for any truck. Does anybody know of a divorced/secondary/external overdrive transmission that I might be able to use for this truck?

3) I seem to remember hearing about 2 speed rear ends. This may be as good or better than the auxiliary trans idea. Does anybody have any info on this option?

Any information is greatly appreciated. Thank you all in advance!

 

I'll call Stewart and Stevens Monday morning and see what they say. Thanks!