twin stick ratios

I think if you replace the 13% under drive splitter in the currrent 13 speed with a 13% overdrive splitter gear from the older RTO 13 speed, it would be the same effect. Eaton sells them to convert a 9 speed direct into a 13 speed overdrive with an .86 overdrive gear. Just slap that kit onto a 9 speed overdrive and bada bing, instant RTOO

 

I think I'd change the rear ends. We run 88k-92k here and most of the trucks have 370 or 373 gears. While we don't have the same grades in Alabama that you have we have some pretty hard pulls on the secondary roads.

 

Some how I missed the important post of this thread I apologize. An RTOO has a higher starting ratio than an RTLO, that may be hard on the M11. The homemade version does retain the lower starting ratio but has a wide gap between the 2 and 3 gear position in both high and low range. Again maybe hard to live with. My 1750 is a high torque box, hard to find used high as giraffe booger new. The M11 is a lot lower torque so a smaller brownie would work fine, just be careful not all are overdrive. easy way to check a used box sittin on the ground for sale is put it in high gear and turn the input. If input and output match its a direct won't help you any. If the output out runs the input its an over. The smaller ones are alot more plentiful and cheap used.
I will say that the two boxes I am running take a noticeable amount more torque to turn than my one double over did. Even a smaller box you will be in the same trap. I understand your reasoning behind this thought, and that you are getting better millage than the big motors on that run. I know fuel millage is our biggest expense money spent today to save money tomorrow makes sense. Think about life time cost in a different light for a minute. The M11 was not designed to pull that weight every day, also no guarantees that after many days down time and many dollars spent the brownie will help the fuel expense but may hurt it as takes more away from the axles to turn it. So you may go backwards, be harder on the engine have more older used parts to maintain etc. I know you are doing better on fuel right now than most bigger engines on the job which is rare as a rule when doing the same work as a bigger engine. Hopefully you haven't had any trouble yet, but in my experience when you work a smaller engine like that every day its not if but when you have a lot of trouble often. Over the total life time expense of a truck the bigger engines usually win even if beat by millage alone. I am not picking on your truck just wanting to give you an angle to think about. Keep enjoying the better millage you are currently getting as long as possible. Hopefully you see the trouble coming and can find the M11 a new Daddy before it shows out. I think you will have more money in your pocket when the smoke clears that way. I am certain you will have less indigestion.

 

Sounds like you have a truck like my ol Pete, just a natural born good truck. I understand the wondering I do a lot of it. I honestly don't know which way it would go and if you are like us and do all your own work that makes a difference to on feasibility of "spec changes." I have no doubt it is cheaper and better to own than any thing with a doser valve. I mean that don't even sound like a real part, thats just proof that the engine manufacture's are just riggin stuff like us ol hillbillies been doin for years. lol