I currently have an 8 3/4 3:91 sure grip in a 741 case. This is too tall a gear for hiway driving and I don't want to go for an OD setup. Also, I am running a 440, and I have read that the 741 cases are more for small block cars. My questions are:

Is there a table that will tell you how many RPM you will drop per rear gear ratio? Like if I go from 3:91 to 3:73, at 70 miles per hour, how many RPM would I drop? 200 maybe? If I go from 3:91 to 3:55, 250 - 400 rpm maybe? I have an A-833 4 speed, not sure of 4th gear ratio, but I think 1 to 1?

Also, I am sure I would need to know how many splines on my axels and pinion before I order one right? Also, how do I tell if the yoke I have is standard or the large yoke?

Any tips or tricks for the installation? Any gotchas I need to watch out for? Common mistakes?

Here is the item number on ebay that I am looking at 380126195207. This is a 3:55 in a 742 case. Anyone used this guy? He has 100% positive feedback.

Does anyone on the forum rebuild these? I looked at parts to do the rebuild, about $400 for bearings and gears, not including the posi unit itself, not sure on labor, but probably close to the same or more. It just seems like a better deal to buy one done with a warranty than to build one. I have a 742 case, and I think the ebay guy said he would give $200 credit for it. Plus his stuff is buy it now or best offer. I saw him accept a bid for $980 for a 3:73 in a 742 case, all new components. If I got that deal and $200 for my case, that would be $780, I don't know how I could lose?

I also saw this on Craigslist: *3.55/742 CASE POSI, USED, CLUTCH TYPE, EXCELLENT CONDITION, NO ISSUES,INSPECTED, $675

The website for this one is http://www.rumblefishltd.net/ Anyone ever used this guy? For just a hundred more, I could have a new one and be rid of my extra case.

Ideas? Suggestions???

 

try this http://www.csgnetwork.com/multirpmcalc.html (round up the last 2 numbers) hope this will work for you? and for the site i only ask for the 391 complete with pumkin!! j/k

 

For a little more fun try this one:
http://vexer.com/automotive-tools/speed-rpm-calculator

I have seen another one similar (from memory it was a bit better) but couldn't find while at work.

 

Hey guys, thanks a bunch for the RPM calculator sites. That gives me a good idea of where to go. With my current 3:91 at 70mph, the calculator shows 3406. If I go to 3:55, it shows 3092 which is 314 rpm less. It does not seem like much, but anything is better than nothing. As for the 3:91 in the 741 case, I would be happy to give it to someone that could use it once I change it out It works fine, but it does have a howl in it. I am not sure why it is howling, but since it is in a 741 case, I am not interested in finding out, I would rather just get the ratio I want in a 742 case since I have a big block.

Any other info on doing the swap? What do you think about the price?

 

Bumping for more help.... I have swapped 3rd members before, just wondering if there is anything Mopar specific I need to know about. The calculators the guys provided helped me decide on a 3:55 ratio. Now I am figuring out if I should build or buy one already done. Any help would be appreciated regarding the removal and replacement of the 3rd member.

 

have you rebuilt one before?

 

I have nor built a rear end before, but I have swapped a couple out. I have a Mopar guy near by who is willing to build the rear end, but when I look at the cost of parts, I don't see how building one can be cheaper than buying one already done with a warranty. I have looked around a lot, but a 3:55 in a 742 case is hard to find. The best one I found for price is $675 which is a used one, but "checked" out of New York. For not that much more, I can get an all new unit for $980 and a $200 core for my 742 case which is $780 out of my pocket. I will keep looking and post some "Wanted" ads.

On the installation of the rear end....any surprises? I hear from someone that there is some spring inside that I need to watch for, true???

 

no nothing to worry about. its a straight forward job. no surprises. never seen any springs.. accept IN the pumkin assembly

 

Thanks for the feedback. That makes me feel more confident doing the switch. I'll let you all know how it goes.....probably down the road a month or two.....or sooner if the 741 case that howls fails me.