9 Speed to a 10 speed

I don't mean to be a smart ### here. But it might be a lot cheaper to just slow down.

 

Really doesn't matter what transmission you pick so long as it is an OD model. That is if your end game is to lower your top speed RPM. A 13 doesn't automatically mean OD either as in some cases the OD ratio comes from the main box, not the aux section.

 

Check the lengths. I think 9's are the same length as a 13/18, and wouldn't require redoing the driveshafts. The manuals are all available on the Eaton website

 

What model is your 9 speed?
The least expensive way is to find a good gear head. Push the front two gears off the counter shafts and swap them. Also swap the drive gear and the front gear on the main shaft. That probably gives you about 18% OD. Put in a bearing kit and replace any worn parts while there.
I thank that 11.1 Detroit likes a little RPM and won't lug down well like a big Cat. If you get over geared you will find yourself down shifting on every little grade and wishing you were still just humming along in 9th.
13 and 18 spds are more versital.

 

You can find all of the Eaton Fuller Transmission Ratios and models on line. Search that and or things close to that. When you have the one I like it lists every model ever made and every ratio in every gear. Also has the length. There are different length yokes also.
Some models have high speed reverses and low gears. Jumping up over a 14 series will have a lot of unexpected problems.
I did not see a RT12906A right off but if you do as I mentioned you will have RTO12906A.
You can also go to Eaton road speed calculator and fill in the blanks. They will give your road speed possibilities. Have fun and I hope you make good choices.

 

What about swapping the rears out to say, 2.64? Just a thought.

 

If you have the transmission you have converted to an overdrive you may have to just drive it in 8th sometimes. That will be the same as you have now all the time. Just hope you have enough power to pull it in high.
That 9 over transmission was a good match with 3.55's, NTC 400 and 22.5 tires.