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Hi I found a Watson M42-M brown lipe I was hoping someone could tell me some information on it I want to put it in my ram 3500 12v Cummins with the HD NV4500 looking to lower my rpms at road speed and would it hold up under a load
 

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  1. The Watson catalog show the Model 42 as having a 1.49 medium low range, 1.00 direct, and .83 overdrive. Unit weighs 130 lbs and takes 2 qts of oil. Original literature shows they could be used in trucks up to 48,000 GVW. Must be awfully strong. Spec sheet was showing them used behind a GMC 3-71 and 4-71. Just curious, are you running a very low ratio differential? Your NV4500 already has a .73 OD. Adding the auxiliary of .83 will pull you down to .61.
 

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I’m running a 3:54 with 30 in tires at 75 mph I’m turning 2200 rpms I wanna bring it down around 17or1800
 

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That should put you exactly on the spot. Just have to remember those auxiliary transmission are not synchronized so shifting can be a bit tricky, especially down shifts. As far as strength, a 4-71 Detroit NA had 160 HP and 400 lb ft of torque. That just happens to be the same as the Cummins 5.9 6bt in the Dodge unit they got the 215 HP unit which had 440 lb ft. That auxiliary should have not problem in that power range. Not sure there was ever a torque range listed in their literature. Those units were way overbuilt. Even your NV4500 isn't rated all that high. It's basically a NP435 with an OD gear added on.
 

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I ran a Watson with the .73 OD behind a Getrag with 4.11s. Purred right down the road.
 
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