Rear end Gearing

[Cudajason]

All right folks, while I am still knee deep into my Hydro Boost project, I am already thinking of my next upgrade.

The one component left on my car that had never been touched is the rear end.  I have owned the car for over 30 years and it has never been apart.  There is a small fluid leak on the pass side, both other then that it has also never given me any trouble, but I think it is next on my list to upgrade. 

I am currently running an open 741 unit with a 3.23.

I am think of getting a new 489 case sure-grip unit from DR Diff, because who else would we deal with!.  I am debating between 3.91, 3.73 or 3.55 gears.

Thee car will be used mainly around town and back road cruising, but I also want to be able to jump on the highway and go to Carlisle in the future. 

I am running a 42RH OD trans and a 28 in tire. 

Any thought's on the best gearing?

I am also going to pick up new bearings etc. while I am at it, basically overhaul the whole unit!

[anlauto]

I would say a 3:73....I've been using that behind the Tremec 5 speeds, and it's nice...

[Brads70]

I'd vote for 3:91 at 2000 RPM's you will be doing 62 MPH , with that first gear ratio of the 42RH you will need more gear ratio getting off the line.

https://www.blocklayer.com/rpm-gear.aspx

[dodj]

I'd vote for 3:91 at 2000 RPM's you will be doing 62 MPH , with that first gear ratio of the 42RH you will need more gear ratio getting off the line.

3.91 if it were me.
Maybe even a little more...4.11.....but would have to do the rpm calculations for the highway to see if it's still reasonable.

[Brads70]

I'd vote for 3:91 at 2000 RPM's you will be doing 62 MPH , with that first gear ratio of the 42RH you will need more gear ratio getting off the line.

3.91 if it were me.
Maybe even a little more...4.11.....but would have to do the rpm calculations for the highway to see if it's still reasonable.

4.11 only drop the speed to 59 MPH at 2000 rpm's OD transmissions need more gear than most are used to . I think you will find better fuel economy also with 3.91-4.11 as compared to 3.23

[anlauto]

I guess the automatic makes a difference 

[Mr Cuda]

This is my favorite pet peeve topic.  4 .10's so I'll  throw an overdrive in!!! Lol.
The factory stopped at 3.73 for a reason.  27" tire and overdrive is 2200 at 70.

Lower gear ratios will build heat in the diff, so plan on adding cooler.
Lower gears spin the driveshaft faster and build harmonics in the chassis.

Engine; hydraulic flat tappet lifters  need 2200-2400 to have sufficient splash oiling to prevent scuffing.  Factory went to roller to avoid this. Modified engines have no peak torque below 2600 and get worse mileage cruising at that speed.  Massive response problems with a carb, so get fuel injection.
Did 2 back to back 4000 mile trips in my 340 Cuda. First trip with 3.23's @4000 rpm. Got 14.5mpg
Thought I could do better on next trip, and put 2.76 in.  No power and milage dropped to 12. Put 3.23's back in, back to 14.5


Transmission;  standard hp converter stalls at 2400 to 2500. More if against modified or stroked torque motor.
At 2200, how much response will your engine have? 
Now, you will have lockup,  but not till it shifts into od, so you will need a high stall lockup converter,  which are bucks
My cop car with 2.94s had to be driven in second at any speed less than 60 as the tach would read higher in third.
Burned that trans up quick, replacement is still going.
Didn't think the OD trans fit the tunnel?

So even if you gear it to cruise at a rpm range your engine likes,  can you correct all the other issues that will pop up? I can go on....

[gzig5]

As others have mentioned, with an OD trans I'd go with at least 3.91 if not steeper.

[dodj]

Lower gear ratios will build heat in the diff, so plan on adding cooler.

My 6.7 cummins truck with a 410 gear hauling a 12k trailer never needed a cooler...doubt a cuda would.
But I'm curious. Never seen a 489 diff with a cooler. Have a pic?

[Mr Cuda]

Lower gear ratios will build heat in the diff, so plan on adding cooler.

My 6.7 cummins truck with a 410 gear hauling a 12k trailer never needed a cooler...doubt a cuda would.
But I'm curious. Never seen a 489 diff with a cooler. Have a pic?

This is exactly why I bring this stuff up.
I'm sure your newer dodge has 32" tall tires, maybe more.
That brings the effective driveshaft rpm to 2583 @60.
Same as a 3.23 with 26" tires.
Add in the fact that you have a dana 70 or 80 with a 10" ring gear.  More teeth, less heat.

[dodj]

But four times the weight..

[Cudajason]

This is my favorite pet peeve topic.  4 .10's so I'll  throw an overdrive in!!! Lol.
The factory stopped at 3.73 for a reason.  27" tire and overdrive is 2200 at 70.

Lower gear ratios will build heat in the diff, so plan on adding cooler.
Lower gears spin the driveshaft faster and build harmonics in the chassis.

Engine; hydraulic flat tappet lifters  need 2200-2400 to have sufficient splash oiling to prevent scuffing.  Factory went to roller to avoid this. Modified engines have no peak torque below 2600 and get worse mileage cruising at that speed.  Massive response problems with a carb, so get fuel injection.
Did 2 back to back 4000 mile trips in my 340 Cuda. First trip with 3.23's @4000 rpm. Got 14.5mpg
Thought I could do better on next trip, and put 2.76 in.  No power and milage dropped to 12. Put 3.23's back in, back to 14.5


Transmission;  standard hp converter stalls at 2400 to 2500. More if against modified or stroked torque motor.
At 2200, how much response will your engine have? 
Now, you will have lockup,  but not till it shifts into od, so you will need a high stall lockup converter,  which are bucks
My cop car with 2.94s had to be driven in second at any speed less than 60 as the tach would read higher in third.
Burned that trans up quick, replacement is still going.
Didn't think the OD trans fit the tunnel?

So even if you gear it to cruise at a rpm range your engine likes,  can you correct all the other issues that will pop up? I can go on....

wow that's a lot of information man.  I am not sure I follow your concerns.

So what are you suggesting, not to go any lower then a 3.73 gear with a 28 inch tall tire, because that is what the factory did?

[Cudajason]

As others have mentioned, with an OD trans I'd go with at least 3.91 if not steeper.

Thanks for all the responses folks.

I like the idea of the 3.91s, a little more oomph off the line would be fun.

I'd vote for 3:91 at 2000 RPM's you will be doing 62 MPH , with that first gear ratio of the 42RH you will need more gear ratio getting off the line.

https://www.blocklayer.com/rpm-gear.aspx

I use the same site for RPM calcs, its cool.  I think 2000 at 62 mph / 100 KPH is not too bad. 

Going to 4.10s only adds another 100 rpms so.  That sounds doable too.

Cruising on the highway around 70 mph / 110 120 KPH at under 2500 rpm, seems ok. 

The main thing holding me back is my experience last summer cruising two line highways at 60MPH turning 1600 rpm.  Man it was such a great drive, car just floated down the road, the engine seemed very happy, did not lug at all.  that being said, the acceleration is never that great, as expected with 3.23s.  So I would love to have neck breaking acceleration.  More then likely that is where the car is going to live, street vs the highway. 

 

[jimynick]

"So I would love to have neck breaking acceleration."  LOL and it'll pass anything, but a gas station Jas. I had 3:91s in my Valiant with a 4 speed and at 70 mph, it was wailing. I know you've got that OD in it and hopefully that'll make the difference. AND, saying 2000-2200 rpm @60 mph isn't too bad, makes me think of my F150 that turns about 18-1900 rpm at 100k+. However, as you said, you use the car recreationally and so, damn the torpedoes, full speed ahead! 

[Mr Cuda]

This is a subject I am very passionate about.
  Reading your post , the 1600 rpm at 60, is not possible with 28in tires and 3.23, according to the rpm/gear/ tire charts I have.
Even so, it brings up the camshaft speed question.  And converter flash issue.
Rpm is not the enemy.  Proper tune can be efficient even at higher speeds. When posting my mileage,  my average speed is 85, and I am traveling through mountains with 100 to 4000 ft elevation changes.
Driving at 70 with 3.91's  overdrive or not will probably ruin the gearset in short order if you were driving 600 or more miles a day, like on a road trip. For an eye opener, (all you people driving with 3.91) get a pyrometer and check diff temps after a 100 mile trip. Or 40.
I've stuck with 3.23 and built my car around them. Wide ratio 4 spd (3.09 first ) 245 60 15 at approximately 26.5 in tall. 340 , holley 780. Fun car.