Matching transmission to gears help

Overdrive 9 speed won't help that much, and I suspect you have one already. 4.42 is slow, even slower than 55 mph slow.

3.90s are, unless you need low gears, are still pretty slow. At 65 with a 13 speed and 11r24.5, I'm at 1,600, which is fine for 12.7 Detroit, but I wouldn't wind a big motor like that.

 

Just buy a 4 speed brownie and hang it behind your current transmission, you will have perfect gears for every conceivable situation.

 

If I were able to change transmissions and rearends, I would put in a 15 speed and 3.90s. With that Cummins, I'd probably be willing to buzz it a little more than my engine, but not that much. That would get your highway rpms down and get your speed up, but you'd still have your off road granny gears.

 

Hauling heavy with a 290 Cummins. He might just need all those 4:42 can offer.

Also don't those older big cams like to sing. Sounds like its geared perfect to me.

If anything get rid of that 9 speed and put a rtlo18 in there. Be perfect.

 

It is an auxiliary transmission that typically has four gears:

1. Lo lo
2. Lo
3. Direct
4. Overdrive

This would allow you much more reduction starting out, and an overdrive to help out with highway speeds.
I am not sure that it would be the most cost effective solution though. As Swaan said, if you can replace it with a double overdrive 18 speed (.85 and .73) it will help out a bunch. With your rears a 13 speed will work as well.
Your shiney 290 probably needs the rears that you have, and I would say that you want to run it from 1700 to 2100 anyway.

 

Something's not right here.
According to my calculations:
rtx12609B has a .73 overdrive
11R22.5 tires have a revolution per mile of 498
2000 RPM x 60 (revs per hour)=120,000
120,000 /.73 = 164,383.56
164,383.56 / 4.42 = 37190.85
37190.85 / 498 = 74.68 mph

The Eaton road speed calculator ( Calculators )
does not list a 4.42 ratio, but with a 4.33 you are @ 76.2, and with 4.56 you are @ 72.4. @ 2000 RPM.
With a direct transmission you would be around 52 mph with 4.42 rears and 11R22.5 tall tires.
An RTX 12609P is a direct drive transmission. Is it possible that that is what you have?