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What rear gear ratio should i do on rebuild?

This is a discussion on What rear gear ratio should i do on rebuild? within the LT1 forums, part of the Vehicle Specific category; I'm going to be doing an overhaul on my differential and was wondering what ratio you guys recomend for M29 ...

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    What rear gear ratio should i do on rebuild?

    I'm going to be doing an overhaul on my differential and was wondering what ratio you guys recomend for M29 transmission? 3.55, 3.73, 3.90 or custom? Just for info. I'm doing all internals of it (major overhaul kit) c-clip eliminators and strange axles.

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    I did a little looking and it appears that 3.73's are the most common recommendation. You can find some on-line calculators and input different gear ratios to see your highway rpm and such -- pretty simple to do.

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    You can put stronger axles in the 10 bolt, but you're still dealing with a small ring gear and pinion, especially when you change gears to a higher (numerically) ratio. It's still going to break at about the same point a stocker would. As far as the ratio, it depends on what you want to do with the car, i.e what are the goals and how do you want to change the performance.

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    I want to go faster through the rpms, but still maintain daily drive-ability. The thing is yea 1st and 2nd are higher than the mn6, but my 3rd is a 1.34 or 1.35 (have to look at slip, custom modified from factory) not 1.44 so its im not sure if i want to go closer to the 4.11 with the 3.90 or a custom ratio. I have the calculators on a excel spreadsheet that take into account rim and tire size and all kinds of good stuff, but its still a hard decision to make. anyone driven an m29 with 3.90s that could throw in some input? I really Didn't want to spend the extra grand for a 12 bolt but it seems everyone out there agrees the 10 bolt just won't hold up. anyone have any luck with a junk yard 12 bolt then modified? Or should i quick fu**ing around and cough up the dough for the Moser?

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    FWIW, I have a Moser 12 bolt with 3.73's in our car and it is a bit on the noisy side.

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    Quote Originally Posted by 93TransAm View Post
    I want to go faster through the rpms, but still maintain daily drive-ability. The thing is yea 1st and 2nd are higher than the mn6, but my 3rd is a 1.34 or 1.35 (have to look at slip, custom modified from factory) not 1.44 so its im not sure if i want to go closer to the 4.11 with the 3.90 or a custom ratio. I have the calculators on a excel spreadsheet that take into account rim and tire size and all kinds of good stuff, but its still a hard decision to make. anyone driven an m29 with 3.90s that could throw in some input? I really Didn't want to spend the extra grand for a 12 bolt but it seems everyone out there agrees the 10 bolt just won't hold up. anyone have any luck with a junk yard 12 bolt then modified? Or should i quick fu**ing around and cough up the dough for the Moser?
    Dont try to modify a junkyard axle, you'll end up spending near the same or more, and it'll still be used stuff. Also, a 12 bolt isnt really the best axle out there, but it will work. The dana 60 is especially strong and has the lowest parasitic drag. A 9 inch is also strong with the ease of easily changing gears, but higher power loss.

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    isn't the dana 60 the heaviest axle though? I don't want to do ford 9" cuz we had a bad almost impossible shimming problem with the last one and endedup going 12 bolt instead for ease of instal. although I am building it for strength and longevity so i might want to go with the dana despite the weight. I also decided to go around 450hp with the motor build so it'll last. the last build was for longevity and we got 201k out of 406hp. so i figure 150k out of 450 should be feasible. still no-one whos gone 3.90s with an m29 though? or knows anyone with it?

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    Strange makes a "lightweight" Dana they call the S60. Only supposed to be 25 lbs. or so heavier than a 12 bolt.

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    Quote Originally Posted by 93TransAm View Post
    isn't the dana 60 the heaviest axle though? I don't want to do ford 9" cuz we had a bad almost impossible shimming problem with the last one and endedup going 12 bolt instead for ease of instal. although I am building it for strength and longevity so i might want to go with the dana despite the weight. I also decided to go around 450hp with the motor build so it'll last. the last build was for longevity and we got 201k out of 406hp. so i figure 150k out of 450 should be feasible. still no-one whos gone 3.90s with an m29 though? or knows anyone with it?
    Yes the 60 is the heaviest axle. Dont discount the 9 inch just because you had a shimming problem with one. Have a pro set up the gears. It's the best option if you plan on changing gears at any point, or changing often. I'd still go with the 60 if you are sure on your gear ratio.

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    I put an S60 in my T/A two months ago. I have a A4 and I went with 4.10s, I wanted to go 3.90s but they are not available with the S60. I have only put maybe 100 miles on it so far but it does have a slight whine when you let off the gas. I have heard that the whine goes away after the first oil change. On the highway at about 70mph my rpms are around 2400, I am still tuned for the stock gear so that might change after the new tune. The S60 fit like a glove but you will have to get a shorter driveshaft, I just ordered one from Strange and it was shipped with my rear.

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    The S60 is bullet proof. It may weigh a little more ( 25 lbs. ) but you will never have to worry about the rear again, even with slicks. Spend the money not and you will save in the long run.

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    That is why I went with the S60, I will never have to worry about breaking it and I will never have to upgrade.

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    Well i guess that I'm going to have to wait another month or so and save some extra cash to get one. Thanks for the help guys.


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