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okay so i am in a bit of a bind, i am doing a cumins conversion into my 1990 crew cab long box dually. i know i have a 14 bolt in the rear but not to sure about the front i am pretty sure it is a Dana 60 but not positive ?. i am putting a Allison behind my cumins but to get my gearing right for usage on the highway i am wanting to use a 3.42 ratio needing help to find out if i can use that ratio in the front axle as well ???
 

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If it is a 90 square body dually, you have a dana 60 up front. IIRC a 3.42 ratio is not available for that axle. What size tires are you running?
 

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is it a 14bolt or dana 70 in the rear?every dually of that vintage I have seen has a D70
does the allison not have overdrive?

you can get 3.73's for the D60,14B and D70
you can get 3.54's for the D60 and D70 but not the 14B
if you swap the front to a D61 from a 89ish Dodge and matching D70 you can get 3.07's
 
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