2.76 to 3.91's SG

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MuuMuu101

MuuMuu101

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I have an A body 8 3/4 I'm saving for my Dart sitting in the corner of my garage. It currently has an open 2.76 gears with a 489 case. I got a lead on a set of 3.91's with "less than 500 miles on it." The guy says it has a sure grip and says it's "the big case (not 741)." How would I validate it to see if it's a 489 case? He has it listed for $500 shipped. I wouldn't buy it without seeing pictures of the gear teeth. Is this a good deal? Also, How bad would it be driving 3.91's on the highway at 60 mph with a 25-26" tire? My guesstimate says I'd be running around 3250-3500 rpm. The Dart has a 273 and a 904. Would like to go to a 360 and some 4 or 6 speed manual trans in the farther future. Is changing gears something that can be done in the garage? Thank you.
 
There are three different carriers. 741, 742, 489. This is determined by the last three digits of the casting number. If it is a clutch style unit it is well worth the 500 and more. If it is an auburn cone style that would be top dollar for a used unit.
 
For what it is worth I had 3.55 gears with 25 inch tires and bought 2.76 gears for the highway. With the 3.55 gears I was at 3000 rpm's at 60 mph which I did not like on long trips. I now change gears depending on what I will be doing with the car. JMO
 
You can easily change the gears in your garage. The hardest part is hoisting the center section up into place on your back.
 
guess it depends on your driving ..if you drive the highway for any long distance I would wait till you go with the 6 speed to run 3:90 or lower type gears ..there very nice on the street but a bit much if your on the highway a lot ..and with gas at $4 its expensive cruising.... changing rear gears not big deal other than lifting that chunk as stated ..I know I can't any more ....getting to old and too smart
 
if its a 489 case it will have a cone type limited slip if factory if its a 741 or 742 it will be a clutch type some people call the 742 the stronger case even though the 489 has the bigger pinion stem the strength between the two has been argued if you are concerned about the swap the 741 742 and 489 all will swap into a 8 3/4 housing as for identification all you do is look at the last number of the line of numbers below the pinion if you do get a 489 id spend the money on the solid sleeve kit they make to remove the crush sleeve

here is an example of a 741 case

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quick note i have read some where the 489 has a big 9 on the side and the 741 has a big x on the side and the 742 has a 2 on the side of it

also dont dissregard the 742 its just as much a power house as the 489 on that note i have seen some pretty nasty cars run 741
 
The rear end is out of the car so I can move it accordingly. I'd like to cruise around in it quite a bit. Not just city driving. My current 7 1/4 has 2.94 gears and was a dog sometimes going up hills. Is it really that unbearable. Besides gas mileage, I'd assume the exhaust could get annoying at the higher rpms. I have a stock exhaust with some aftermarket muffler. I just don't know if I should make the buy since I have no idea when my car is going to run again.
 
all i ever used it for was rpm determination but you do make a point speed wise the other one is new to me ill have to compare the two and see how i like the one you found
 
give these guys a look http://www.justdifferentials.com/ they got me a master kit and ring and pinion for just under $400 on the dot i went with 3.71 gears and have liked them so far you can pick up used cone type slips for a good price now and then and if you can rebuild the drop out your self there is a good chance you can hit that $500 dollar mark i had a shop do mine and was only charged $100 for labor
 
moparfury123 said:
give these guys a look http://www.justdifferentials.com/ they got me a master kit and ring and pinion for just under $400 on the dot i went with 3.71 gears and have liked them so far you can pick up used cone type slips for a good price now and then and if you can rebuild the drop out your self there is a good chance you can hit that $500 dollar mark i had a shop do mine and was only charged $100 for labor
Click to expand...

That could be an option. May not be a small mini-project to try out. I definitely would like to learn. :thumblef:
 
Next time you're out for a drive, pull the transmission into 2nd gear and drive around for awhile on the highway. That's pretty much how annoying it will be :D

I have a 3.91 suregrip in my daily '73 Dart, but that's with a 518 overdrive transmission to go along with it.
 
BigBlockMopar said:
Next time you're out for a drive, pull the transmission into 2nd gear and drive around for awhile on the highway. That's pretty much how annoying it will be :D

I have a 3.91 suregrip in my daily '73 Dart, but that's with a 518 overdrive transmission to go along with it.
Click to expand...

That sounds pretty annoying... Lol. I like the justdifferentials idea. Maybe I'd go with 3.55 instead. I would like a T56 but I aint got the time, money or fab skills for that. :p
 
I would run 3.23 or 3.55 if you have not bought the gears yet.
I have 3.23 in my Duster now and they cruise nice.

I also have a fresh set of 4.30 ready if they ever finish the new drag strip they are building that is only 20 miles from my house.
 
I cruise around the country back roads with a 4 speed / 4.10 rear - 3000rpms @ 50mph and it suits me well. As some have said it depends on your driving and expectations.
 
I have had about all the popular gears over the years. 3.23,3.55,3.91,4.10 etc... I've mostly had 3.91 gears in the majority of my cars,but remember I think things were a little different 10 even 20 years ago I think peoples driving habits have changed they tailgate a little more use electric devices, now a days I prefer a 3.23 or a 2.94. But I am also getting older I think I might have slowed down a little. By the way you won't notice to much of a difference between the 55's and the 91's, also people seem to get real hung up on these casing's 741, 742, and 489 good, better , best. Bottom line highly and I stress highly unlikely you will ever break any of them, unless you are making some serious horse power and if you are to would have to get all the traction to the pavement. I ran a '70 Challenger (90's) w/440 4-speed 3.91 741 casing never a problem. Ran a '70 Charger (80's) 440 727 3.91 721 casing never a problem. hopes this helps.
 
Another thing to consider is the torque converter. For instance, a 10 inch 3000 stall converter will also add to the rpm's every time you pull a slight hill. What is easier than changing gears is changing tires. Have two sets of rear wheels and tires. 28 inch for longer drives and 25 inch for weekend warring!
 
I have a 489 case with 3:55 gears SG sitting in my garage. Is there a way to determine if it is good. It is still bolted to the original housing. I took it out of a car that was non running but I did tow it home on tow dolly with rear wheels on ground.
 
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