Help with rear end info/advice... 73 charger/4speed

[euroZ06]

Soooo i got orders to go to germany in June, and id like to take The Red Dragon with me... Due to strict german inspections, there are a few things i need to fix. Oil pan gasket, few mounts, few seals etc... One of the items i dont HAVE to replace, but would probably be a good idea to replace is the rear diff. I have a 440/4speed with 3.91 posi(741 case), and on a highway at 50mph im sitting at 3k rpm(tall tires too). On german autobahns this would be border line dangerous. I have a set of 3.23 open gears (741 case as well), but id really like to have posi. So here is my dilema: keep current gears, and continue being the slowest car on the freeway/autobahn, or figure out a cheap way to solve my problem. A local gentleman has been helping me with this, but i wanted to get another opinion here. He told me 741 case is no good and a few good launches/hard shifts and my diff will blow.

He said it is possible to get my 3.91 diff out, pull posi out(pay a guy $100 to put it into my 3.23), replace all the bearings (a kit for $100++), and he'll charge me for labor $100. In this case it will take a week or two to do it (since he only has one free day a week), and im still stuck with 741 case. He also said my car will be a slouch with 3.23 (well, i should mention he has a 10sec challenger in the garage).

Second option is to find a posi unit, plus seals, plus pay his guy to instal it, but avoid the downtime (he doesnt want downtime as in who knows what can happen). Still 741 case.

Last option is to find a 742/489 case with 3.23/3.55 gears and posi, and put it in, but that can be really expensive or a cat in the bag when buying used.

Or just leave it all as is...? What if im cruising at 4k rpm (should be around 60mph), is that bad on the engine?

Any advice is appreciated. As i said, i have quite a few other things i NEED to fix to take it there, so trying to keep this project at a low $$

[Nacho-RT74]

I'm plenty happy with 489/3.55 suregrip setup on my 74. that makes on my car around 100-110 MPH on around 3500-3800 rpms ( MP 175K TC ). I think enough and still plenty of power from bottom. 27" tires.

true is I have an stroked 400 with 3.75 crank, and nothing fancy extra, midly made... but not way diff from a kinda stockish earlier 440, just higher compression

NOTE... couldn't go faster due a BAD harmonic vibration somewhere on drive line being really NOTICEABLE starting on 110, and getting worse from that... meaning this tailshaft on tranny, driveshaft or... who knows ( yet )

[Alaskan_TA]

Taller diameter rear tires is another option.

[euroZ06]

Well another thing is my transmission gearing. How many different manual transmissions were on 73's? Its not an original 440 (was told it was originally 400, but who knows).

My tires are tall, cant remember the size of the top, but they are tall.

Im not looking to cruise at 100mph (although it'd be nice), just want to do 70-80mph without taxing the engine.

100mph at 3500rpm, r u sure u have 3.55? Maybe auto transmission? Seems like veeery low rpms for that speed. I doubt there is that much difference between 3.91 and 3.55

[1974dodgecharger]

Im at 4k rpm at 110 or so.....and I have 3.23 gearing....he must have a overdrive auto.

[Mike DC]

    
Modern vehicles typically have an axle gear at least 3.23 if not taller.  
And they sometimes have bigger diameter tires than we do.
And they always have an overdrive gear.  
And I'm talking about USA models that aren't intended to cruise much beyond 70 mph.

See where I'm going with this?  I'd vote to get your gearing as tall as you can possibly tolerate.  


The old muscle car world thinks an overdrive tranny makes 4.10 gearing perfectly reasonable on the street, but nobody else thinks that way.  Not in 2015.  The rest of the world has moved on to even taller gearing on the highway (and lower bottom gearing in the trannys to make up for it).      



You might consider swapping in an overdrive 4spd.  There is the factory one from the 1970s/80s.  It is fairly cheap & easy to find on the used market.  It works if you don't abuse it but the strength & design isn't the best.  There is also the aftermarket Passon OD gearset that drops into a stock 4spd case.  It is stronger & better than the stock 4spd OD setup, but the top OD gear is not as tall as the factory OD.

Passon is also making a great drop-in 5spd manual conversion.  But it costs $4k and you aren't likely to get it in 2015, let alone in a few months.  

[Dino]

I really doubt you will hurt that 741 in normal driving with an occassional hard launch.  I also doubt the car will be slow with 3.23 gears.  You couldn't pay me to drive on a freeway with 3.91's and I definitely wouldn't want to be on the autobahn with it.

[Nacho-RT74]

Well another thing is my transmission gearing. How many different manual transmissions were on 73's? Its not an original 440 (was told it was originally 400, but who knows).

My tires are tall, cant remember the size of the top, but they are tall.

Im not looking to cruise at 100mph (although it'd be nice), just want to do 70-80mph without taxing the engine.

100mph at 3500rpm, r u sure u have 3.55? Maybe auto transmission? Seems like veeery low rpms for that speed. I doubt there is that much difference between 3.91 and 3.55

Yes I'm sure unless the autometer tach is wrong LOL.

Remember you are not ALLWAYS 100% pushing out the car with full engine torque, Inertia plays on the game too specially on an auto car and flat highway.

yes of course, auto transmission, I told the TC specks ( MP 175K which is around 2500 stall )

[John_Kunkel]

I have a 440/4speed with 3.91 posi(741 case), and on a highway at 50mph im sitting at 3k rpm(tall tires too).

For that to be correct you'd have to be running 22" tires. I'd say either your tach or speedo is inaccurate. 3.91's with a 28" tire would put you at over 60mph at 3,000 rpm.

3.23's won't make the car a "slouch", 90% of Mopar muscle cars came with that gear.

[euroZ06]

Ill measure my tire, but if memory serves me right its a 15 inch with 70 hight.

My speedo doesnt work, so i use my gps.

Tach is off? It idles where it should (at one point i had idle set at 750 now at 1100), and it seems to be proportionately increasing with speed.

So will 741 case be ok then? My guy said even if i find a 742 case, he cant take out the 3.23 gears out of my 741 and fit them in the 742.

I am having a very hard time finding a set of 3.23/3.55 with posi and 742 case (unless i pay $1500).