Cat 3406E Question! MPG Poor at 3.5-4 mpg.

His reasoning was they’re not likely to damage the deep reduction so you’re basically rebuilding a 10 speed. The 15 isn’t ideal but the deep reduction was nice especially if we were on soft ground. Like anything once you get used to it you just drive the truck. Heck, I also hauled cattle with a 10 speed in my old FLD and people said I’d need a 13 or an 18.

 

I can't really put my finger on why but a 15 is probably one of my favorite transmissions. I had a lot of 80 something model trucks with B model cats fifteens and 370s.

 

So, does my RTLO 16913A have the high side as the auxiliary?
The reason I ask is because another buddy of mine got a “deal” on a 18 speed that I’ve always suspected was a RTX16609B with a 13 speed rear section. With 3.70 gears, he’s easy triple digits. I calculated it one time as he’s right on par with 3.25 gears and his transmission is noisy in the highest gear. He just runs around in 17th all day…..it’s a dump truck also.
ETA: his transmission has the “horseshoe” shift pattern where the third and fourth holes are opposite.

 

I know I liked the small splitsnin the high end for the 13 speed, and while the 9 speed has a deep deep low low gear (1), if i HAVE to use it to get going, im almost guaranteed to have to wind up to 1900 rpm to shift into 1st (2nd) and if on a hill, that wasnt a guarantee i could get into 2nd, we're talking a ratio jump of 12.83 to 7.69

 

That's the holy trinity of drivetrains right there! 40 years later still can't be beat IMO

 

What he probably has is an RTO 14609B with an RTO15618b rear box and main shaft. The B ratio 609 had a really good low hole. You had change main shaft to put a 13/18 speed box on it but what you got was a transmission that had a good low gear and ended in .62 overdrive. With that set up 12th is .73 or same as the newer RTLO style.

You can turn and RTO14613 around but you loose the lower gears. It speeds the whole box up. The RTO16609a has a .73 final but low isn’t as salty. Running that one in direct most times is actually better because the auxiliary isn’t turning. They made a RTOO1413 as well that by the ratio chart really looks like the 14609B.

The RTLO16913a has an under drive auxiliary box. When you flip the big button to high range it also sends supply air to the split button. Since the split button is in direct already when you move the pole to neutral it does 2 things changes your range synchro to high and engages the auxiliary section so that 5L is selected. When you move the split to high it moves the auxiliary drive collar out of the drive gear so power flow does not run through the auxiliary box.

When you move your range button to low it cuts the air supply to auxiliary controls, with no air supply it locks it out so there is no chance of under driving low range. The rear of the 13’s are not made to handle the torque of using it in lower gears. I have seen folks modify their shifter and use 2 push pull valves. That’s fine if you set it up like the factory shifter and it kills air supply to aux in low range. It doesn’t take many times if forgetting and you have transmission troubles.

I decided to put an 18 knob on a 913a one time. Had to plug one port on the slave valve because the 13 and 18 use a different slave and the 18 uses a 3 line knob. It last about 3 weeks when the auxiliary box come unwound and grenaded every #### gear in the trans. From time it went to whining to snapping popping and refusing to move was bout a mile. Expensive lesson

 

I was just watching a TikTok from gearman93 on this exact topic