Just for fun I calculated the gear ratios for the on-road version of the Spicer 6 X 4 two stick transmission, which consisted of a PSO140-6A 6 speed main box with the following ratios:

R: 7.49
1st: 7.49
2nd: 4.08
3rd: 2.26
4th: 1.24
5th: 1.00
6th: .84

And an AMO1750-4D 4 speed auxilliary box with the following ratios

Low: 1.59
Under: 1.24
Direct: 1.00
Over: .81


The low gear in the auxilliary box was used in either 1st or Reverse in the main box. Spicer also sold a "C" ratio version of the auxilliary box that had an extra low 2.37 low ratio.


Here are the available ratios with a PSO140-6A main box and the AMO1750-4D auxilliary box.


Spicer 6 X 4

R low: 11.7819
R under: 9.2628
R direct: 7.47
R over: 6.0507

1st low: 11.7819
1st under: 9.2628
1st direct: 7.47
1st over: 6.0507

2nd under: 5.0592
2nd direct: 4.08
2nd over: 3.3048

3rd under: 2.8024
3rd direct: 2.26
3rd over: 1.8306

4th under: 1.5376
4th direct: 1.24
4th over: 1.0125

5th under: 1.24
5th direct: 1.00
5th over: .81

6th under: 1.04
6th direct: .84
6th over: .68

Obviously some of the higher ratios overlap but it shows the flexibility that the Spicer 6 X 4 gives.

With a 3.55 rear end ratio and tall 11R24.5 tires, 100+ mph is possible while in 6th over!

It's still possible to special order a new 6 X 4 combination, though it's no longer sold by Spicer but a company called TTC (Transportation Technologies Corporation) that took over the production of the transmission division of Dana/Spicer.

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They were tough boxes. I used to drive a Mack tow truck with a mack 5 speed and a Spicer 4 speed aux box mounted mid ship. That truck would really crawl. It ran pretty good down the highway too.

I hear guys used to turn the aux boxes around and end up with a 0.63 overdrive just in the aux. With the overdrive in the main box that would give a total overdrive of about 0.53. That is almost two turns of the driveshaft for every turn of the engine. I wonder how well they held up though.

TTC released a double O/D version of their 18 speed a few years back. It has a .84 and .70 top gears. It's a true double O/D like the 6 X 4, unlike the current popular 18 speed boxes from Eaton that spin the .73 O/D in the main gearbox through a 17% underdrive gear in the aux box to achieve the .86 (8th LO/17th) gear. Consequently Eaton discouraged cruising in 8th LO/17th for extended periods due to potential excessive heat buildup. This applies to the 13 speed models as well.

It's interesting to note that Spicer recommended using double O/D in the 6 X 4 only while cruising empty. I wondered if drivers followed that recommendation.

Eaton did at one time have a true double O/D transmission, the RTOO series.

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I drove An Autocar with an RTOO9513 for McHugh Brothers Crane Rentals Back in the 80's. 4th in the main box was back and 5th was up. It had a 318 Detroit and Eaton 2 speed axles. It was a beast. I think I lost some of my hearing driving that thing.

I never understood why overdrive transmissions had the top gear positions reversed. I guess it kind of fits, the top "overdrive" gear position being "above", or "over" the "drive" gear, lol. Eaton did make an RTXO with the direct shift pattern but I figure those are a rare find.

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The somewhat backwards shift pattern allowed them to use basically the same transmission design. There were just changes in the ratios of the gears between the mainshaft and the countershaft. When you pull the stick back it slid the 4th-5th syncro collar forward locking the input shaft and the output shaft together. The power flow through the trans was then straight through, just as if it had a solid shaft all the way through. With the stick up, the powerflow was going from the input shaft through a gear set to the countershaft through another gear set that was then locked to the output by the collar on the 4th-5th syncro sliding backwards. Power needs to flow through the countershaft in order to get any of the gears lower than direct, or any overdrive gears.

Pretty much what they did was make an RTO and a RTOO that shared a lot of parts.

Actually not - What they now make as an overdrive tranny is called an RTO but in reality they stopped making the RTO and renamed the RTX(O) to RTO just to confuse us all! The newer RTO's have the same shift pattern as the RT's.

now you are talking packrat,i drove a 6x4 in a concrete mixer,i didn't like the long throw in the six x as much as i did in the 4x4 in a kw cabover,you could almost make gears in that thing,but she would fly down the road with a load,just my 2x,seeya.

Heck mine has a 5x3 and will do 90mph # 2100 and has 6.14 gears in the drive axle. I'll post my 2stick vid in a minute if the censor will let me.

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