A question of rear ends.

[carrman]

Guys, I've decided I need to lower the rear gear in the Newport. 2.76 rear gear in a 742 case I have now is rather long legged, but Id like more spirited acceleration than what it gives me. Attached are the pictures of what I'm looking at buying. Opinions on going from a 2.76 to 3.23? Tires are 235/70/15 that are 28" diameter. Most driving is in town with some freeway trips of a few hundred miles in mountain territory. Opinions?

 

[413]

3.23 would be great with those 28”tires. I’m sick of 2.76, it just kills the torque.

 

[Turboomni]

When is everything back in stock? They seem to be out of stock in alot of things for a long while.

 

[HOT FURY]

Personally I'd go with a 3.55 or 3.73 to feel the most difference, you're running a fairly tall tire so should be fine cruising still. I talked to the guys at Dr Diff about a week ago and he said they should be getting in those helical gear differentials within the next few weeks. I'm waiting on one as well, bumping my ride up to a 3.91 with the change, have 3.23's now and not enough kick in the butt for me still. Also unless you're going to go crazy with some torque monster engine I would't spend the money for the 489 case, your 742 case will be fine. If you had a 741 case I'd say make the change.

 

[carrman]

Personally I'd go with a 3.55 or 3.73 to feel the most difference, you're running a fairly tall tire so should be fine cruising still. I talked to the guys at Dr Diff about a week ago and he said they should be getting in those helical gear differentials within the next few weeks. I'm waiting on one as well, bumping my ride up to a 3.91 with the change, have 3.23's now and not enough kick in the butt for me still. Also unless you're going to go crazy with some torque monster engine I would't spend the money for the 489 case, your 742 case will be fine. If you had a 741 case I'd say make the change.

Im looking at building up a spare unit to minimize my down time. I want an in an out swap.

 

[The Goose]

I’d say Stick with 323, 355s suck, they aren’t low or high gear but magically carry the bad habits of both. I don’t think I’d run 391s without an overdrive. They’re pretty hateful out on the highway. Be sure and report back how you liked the upgrade.

 

[carrman]

I’d say Stick with 323, 355s suck, they aren’t low or high gear but magically carry the bad habits of both. I don’t think I’d run 391s without an overdrive. They’re pretty hateful out on the highway. Be sure and report back how you liked the upgrade.

I wish I could find more people who've run the LSD I want to. I do need to know what size u-joints our cars came with.

 

[HOT FURY]

I’d say Stick with 323, 355s suck, they aren’t low or high gear but magically carry the bad habits of both. I don’t think I’d run 391s without an overdrive. They’re pretty hateful out on the highway. Be sure and report back how you liked the upgrade.

mine rarely sees long highway trips but I have kicked around the idea of adding an under/overdrive setup.

 

[Mr onetwo]

this seems like a good start 8.75" 8-3/4" Chrysler Mopar Dodge Nodular Iron Rearend 742 Case - 3rd Member NEW | eBay

 

[WOT440]

I used Mopar Madness, for my third member. Excellent to deal with, older gentleman, with over forty years of experience.
Fast service, and the rear works perfect. I kept the 2.76 as I was looking for comfortable highway cruising.

| eBay

Mopar 8 3/4 8.75 2.94 3.23 742 OEM Used Reman OPEN, Add Posi New SURE-TRAC Dodge | eBay

Mopar 440 Engine Auto Parts and Stroker Motors for Sale at Mopar Madness

 

[MONC440]

Going from a 2.76 to a 3.23 you are going to feel the difference. If you truly drive a lot of city driving and only minimal hwy a 3.55 would be an option anything above that you will probably be kicking yourself in the a$$ when you do have to drive on the hwy.

Just an FYI:

Here are some numbers for different gears at hwy speeds with a stock 727 and 28 inch tall tires.
2.76 @ 70 = 2380
3.23 @ 70 = 2780
3.55 @ 70 = 3060
3.73 @ 70 = 3220
3.91 @ 70 = 3370
4.11 @ 70 = 3545

I have had most of these gears in different cars with different engines and tire sizes and IMO I try to stay under 2800 rpm on the hwy with 413 and 440, under 3000 with 360 and 383 and under 3200 with 273, 318 and 340.

 

[57fury440]

Depends on the speed you like to drive at on the highway. At 60mph the 3.55 would put you at 2624 RPM's. For what you want I would go with the 3.23 gears. The 3.55 gears would make a huge difference from what you have now but is less friendly on the highway.

 

[MONC440]

Depends on the speed you like to drive at on the highway. At 60mph the 3.55 would put you at 2624 RPM's. For what you want I would go with the 3.23 gears. The 3.55 gears would make a huge difference from what you have now but is less friendly on the highway.

You are right but here in Michigan if you are driving 60 on a major hwy you are a traffic hazard. Plus you give all the old cars a bad name because everyone blowing by you will think old cars can't keep up. LOL

 

[The Goose]

I ran 391’s in my AAR daily for 10 years. It was great from a dig and even with a flying leap it was unpassable when rolling. Back when fooling around on the street wouldn’t net you the death penalty we’d let them get even with the back bumper and then keep em there. Long trips like Phoenix to payson were great as you could pass a line of 5 10 or 20 cars going 45 uphill with ease. Those gears MADE that car. For long drives we just turned up the radio and bought more gas than most cars. Was it worth it - hell yes. Would I do it now - you bet but I’d run it with that 5 speed A833 they came out with a few years back. With the 14 gallon tank at 6 to 8 mpg we did about 90 miles per tank and always carried a Jerry can for backup.

I planned my trips based on gas stations. Do it to it!!!

Forgot to say rear tires were 275 60 15 if that helps.

 

[330dTA]

Back in the day, my godfather had similar Javelin SST 360 as the one above. Boy did it fly. I loved it.

 

[57fury440]

I have 3.91 gears in my "57. It has a 4 speed and I love those gears however I do not do a lot of highway driving. You would hate them for that purpose.

 

[70bigblockdodge]

Those rpm charts are nice and they make everything rosie. If you have a very newer stock 12" converter your slip will be 2-4%, any converter smaller and higher "stall" number you are easy 5-7%. This is why 3.55 gears suck. Any car without a overdrive I would not go lower than a 2.94 with today's modern traffic speeds. If all you ever do is cruise around suburbia maybe, but what fun is that?
7260 small u joint <2 1/2" across inside of yoke
7290 large u joint> 2 1/2" across inside of yoke

 

[413]

Measurements across the inside of the clips
7260 is 2 1/8” decimal 2.125
7290 is 2 5/8” 2.675

with 28” tall tires 3.23 is fine. But 26” tire needs 2.94

 

[Gerald Morris]

3.23 definitely optimized for CITY driving. Even w a busted block 383 last Fall, one week with 3.23 gears convinced me that in town, this is an ideal ratio! Not so on the highway, but I don't get out on the open road much while tethered to my spawn.

 

[thrashingcows]

I've switched between the 2.76 and 3.23 in my 68 T/C wagon in the past, I would drive hundreds of Hwy miles on vacation in the wagon and thought the 2.76 would net me better MPG and cruising...didn't find that at all. The 440 would bog down on the hills and you would have to drop into passing gear to hold your speed on the hills, so with all that down shifting and higher RPM's the MPG's were actually worse then the 3.23's.

After that I put the 3.23 center section back in and have never looked back...perfect all around gearing for a big block in a C-body, decent acceleration and great Hwy cruising!

As for the U-joint the 742 vs 489 run different sizes, but they do make a conversion U-joint with the 742 on one side, 489 on the other....I've used it in the past with no issues.