Rear end gearing help needed

[DodgeGuy]

So currently my '74 Challenger Rallye is running 3.91 gears. One thing I've learned quickly about this gear ratio is that while it gets off the line like the proverbial bat out of hades, it's not a fun highway drive, as it revs the engine up way too high.

Now, as I've said here before, 25 years ago I knew a lot about these cars, but with time a lot of that information has eroded.

I was thinking that 3.55 gears might be better, but as I've done a bit of reading online of various websites, etc., it sounds like a 3.23 gear ratio (which was the original stock gear) might be a better option for what I want to do.

My "strip" days are long gone. I have no desire to take this car to my local dragstrip and run 1/4 miles. What it's for is local parades, and local car shows. I live in a VERY small town, so in all honesty, this car is going to be driving from town to town (meaning highway driving) more than in town driving (and again, dragstrip driving will be zero). From what I've gathered, the 3.23 gear would maybe be the best blend of take off quickness with top end speed (which for me is more like 60 MPH) without revving the engine so much.

I'd appreciate any advice on this. After researching, I realized there might be a lot more to think about as people are asking about tire sizes, etc.

The 360 4 barrel HP engine is nowhere near stock anymore, and is PLENTY powerful. I'm not sure what all has been done to it, as I bought the car this way. What I can see is that it has headers, an aluminum high rise intake, an Edelbrock carb with duel fuel feeds (a double pumper?), it has an MSD 6AL ignition box, and an MSD Blaster 2 ignition coil. It definitely has some type of bigger cam in it, from the way it runs. I have no idea, other than the cam, what has been done internally, as far as heads, crank, boring, etc.

It has the original factory 4-speed manual transmission.

That's all I have gentlemen, please let me know what I need to consider, etc.

 

[DetMatt1]

What size tires are on it now?

 

[DodgeGuy]

What size tires are on it now?

Front: 225 70 R15
Rear: 275 60 R15

 

[AUSTA]

I run a 3.23 (340 , quickfuel ,point & roller lifters )with 245/60/15 rear done 200 miles a couple of weeks ago sat on 60 mph at 2200 to 2350 rpm.
With the TA i have 295/50/15 with a gear vendors OD so on long runs it is the same as the 3.23 about 2400 RPM
at 60 - 63 mph.
The 3.23 is not race car out of the hole but still has good torque for take off & overtaking.
tried it with 255/60/15 only a couple of hundred RPM difference .
Didnt like the look of the 255/60/15 seems too tall on the challenger

 

[DodgeGuy]

I run a 3.23 (340 , quickfuel ,point & roller lifters )with 245/60/15 rear done 200 miles a couple of weeks ago sat on 60 mph at 2200 to 2350 rpm.
With the TA i have 295/50/15 with a gear vendors OD so on long runs it is the same as the 3.23 about 2400 RPM
at 60 - 63 mph.
The 3.23 is not race car out of the hole but still has good torque for take off & overtaking.
tried it with 255/60/15 only a couple of hundred RPM difference .
Didnt like the look of the 255/60/15 seems too tall on the challenger

So it sounds like you like the 3.23 gears for the highway? From what I've read, going from 3.91 to 3.55 wouldn't make that big of difference for highway driving, it sounds like the 3.23 is the way to go.

 

[moparlee]

Engine RPM Calculator

Here is an engine RPM calculator based upon final gear ratio, tire diameter, and transmission output ratio. There are other calculators out there, but are all basically the same.

 

[david411]

I have a 70 Challenger R/T 440/6 and I went from a 3:55 to a 3:23 for just the same reason. I love it. I needed to pull the chunk anyway because of a slight whine that I didn't like.

 

[DodgeGuy]

So I just found a guy who is selling an 8 3/4 posi rear end with 3.23 gears...in a 741 case. He's selling it for $600.

First of all, I know my Challenger has the 8 3/4 rear end. How can I verify that this will fit in my Challenger?

If I go look at this, how can I verify that they are indeed 3.23 gears and a 741 case? How about verifying that they are posi traction?

 

[AUSTA]

The last 3 numbers on the side will tell you eg: 741,742 & 489
Note not all gears are interchangeable between the models the pinions are different.

 

[DodgeGuy]

The last 3 numbers on the side will tell you eg: 741,742 & 489
Note not all gears are interchangeable between the models the pinions are different.View attachment 45643

Thank You Austa, so basically I need to look at mine to see what housing number it is, and if it's a 741 I know it should be a match, correct? As far as telling what ratio the gears actually are, it it stamped somewhere that will allow me to tell if they're actually 3.23 gears?

 

[AUSTA]

The other chunks 742 , 489 will fit the 8.75 housing as long as the axles splines & drive is the same.
The ratio will be on the tag shown in the pic ,the best way to check is the ratio it should be stamped on the circumference of the crown gear.

 

[DetMatt1]

So I just found a guy who is selling an 8 3/4 posi rear end with 3.23 gears...in a 741 case. He's selling it for $600.

First of all, I know my Challenger has the 8 3/4 rear end. How can I verify that this will fit in my Challenger?

If I go look at this, how can I verify that they are indeed 3.23 gears and a 741 case? How about verifying that they are posi traction?

That seems kind of steep for 3.23 in a 741 case unless it has been rebuilt or it can be proven to be bolt-in condition. I just replaced the 2.76 741 that was in my ‘71 with a 3.23 489 and it required no other mods, just took the old one out and put the new one in after I had it rebuilt.
To be honest I hope I like the 3.23 better after it gets some taller tires. I couldn’t imagine driving one of these with a 3.91 in it. I sometimes regret not rebuilding the 2.76 and reinstalling but it wasn’t a sure grip...

 

[CARPORT T/A]

You would also need to verify what yoke you have on your car... 7260 vs. 7290. If you have a 489 case do not change the yoke. There is a crush sleeve that has to be replaced or the pinion nut will come loose and ruin the gears. You can change the yokes on a 741 and 742 case no problem, as long as they are the same spline (course vs fine). If there is a difference in yoke sizes you can just run a conversion joint, 7260x7290 and you're good to go. Moog makes one that is carried at most parts houses. On a street car I wouldn't be afraid of a conversion joint, but on a race car I would be... I have 275/60/15's on my T/A Challenger with 3.55s and an automatic and it drives down the road nice. Since you have a 4 speed there is no converter slippage like an auto car has. I have run every combination over the years and my favorite overall street gear is a 3.23 for an automatic car and a 3.55 for a stick car. I currently have 2.76s and a 30" tire on my 540 cu.in. max wedge car for street driving and it runs down the freeway at 80 MPH no problem. Going from a 3.91 to a 3.55 will only change your RPM on the road by about 250. Going from 3.91 to 3.23 will lower your RPM by about 500. Get all the ratios(2.76, 2.94, 3.23 and 3.55) and try them out, see which one you like. Only takes about an hour to change them out...