***-End Dilemma

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Captainkirk

Captainkirk

Old School Mopar Warrior
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Hey, folks, building my '72 340 Duster and I have the option of using 2 differentials. First one is the one that came with this car, a '72 340 Duster with factory A-833 4 speed. Not sure of the gear ratio but definitely less than 3.91. It's an 8-3/4 diff.
Option 2 is a 3.91, also an 8-3/4. This one came from my first car which came from the factory with a 340 3 speed manual. The difference in drive shaft diameters is pronounced with the 4 speed shaft being much larger (beefier) than the 3 speed unit. Visually the rear axles look the same, but the 3.91 has around 42k miles and the other one has over 100k on it. Is linking up the 3.91 gonna limit my top end much? This is planned to be a fun street terror car, not a daily driver or anything, so 100mph runs on public roads could get expensive, if you get my drift. Not planning on racing, maybe a one-time run just to see ET. Maybe a local car show now and again. Which ***-end would you recommend, and why? Decision time is right around the corner so need to get my ducks in a row for cam selection, etc.
 
A 3.91 is pretty steep. What size rear tires are you running? From there we can figure the engine rpm at different cruising speeds. @AJ/FormS is handy with those numbers.
 
It all depends on your rear tire size. Taller tires reduce actual gear ratio mathematically, I believe gear ratios are figured with 26" tall tires. 3:91's will not be friendly on a long highway run. Around town they are a fun gear.
 
toolmanmike said:
A 3.91 is pretty steep. What size rear tires are you running? From there we can figure the engine rpm at different cruising speeds. @AJ/FormS is handy with those numbers.
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Thanks for the quick response!
Haven't gotten anywhere near to choosing tires yet. Looking to probably going fairly close to original stock size (Rallye wheels) but open to suggestions. Rubber is 40 years+ old so not planning on re-using what's on there now. I was born at night but not last night! LOL
 
Making me wish I had a 3.09 low gearset in my 833 and a 355 or 373 rear ratio
 
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What other 8-3/4 rear end ratios were factory options in 1972? Trying to determine the gear ratio of Option #1. Definitely 3+
 
3.23 was the most common. 3.55 was a option. Without looking it up I don't think you could get a 3.73 or 3.91.
 
toolmanmike said:
3.23 was the most common. 3.55 was a option. Without looking it up I don't think you could get a 3.73 or 3.91.
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I'll get out in the garage tonight and give it a spin through. I should be able to tell if it's 3.55 or 3.23 visually. Any ID marks on the chunk OD that would identify what gears are actually inside?
 
Is the rear still in it? You can do this. I'm guessing 323 I feel lucky today.
So an easy way to determine your actual gear ratio is to check the tag attached to the differential cover by the cover bolts. On the tag there should be some numbering such as 3.54 or 3.73, either of those numbers will give you the stock axle ratio. To verify, or to see if that ratio has changed, you can jack up the rear axle and support it with jack stands. With the transmission in neutral, rotate the rear driveshaft by hand. Count the number of driveshaft revolutions required for one revolution of the tire.




If the driveshaft turns approximately 3-1/2 times, you have a 3.54 ratio, etc. It is helpful to use a grease pencil or chalk to make a reference point on both the tire and driveshaft while performing this test.



Number of Turns Approx. Gear Ratio
3 3.05 : 1
3 - 1/2 3.54 : 1
3 - 3/4 3.73 : 1
4 4.10 : 1
4 - 1/2 4.55 : 1
 
Captainkirk said:
I'll get out in the garage tonight and give it a spin through. I should be able to tell if it's 3.55 or 3.23 visually. Any ID marks on the chunk OD that would identify what gears are actually inside?
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After so many decades, even if a tag is on there, it might not be what is actually inside.
If you have a sure grip;
get both wheels off the ground. Mark the lowest point of the tire and the driveshaft, both against fixed reference points. Then rotate the tire exactly 360 degrees while counting the driveshaft revolutions, which will be equal to the ratio.
If you have an open diff, jack up only one wheel off the ground, and now the driveshaft will turn twice as many revolutions, so divide the result by two .

OOPs treed by @Kendog 170
 
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Captainkirk said:
What other 8-3/4 rear end ratios were factory options in 1972? Trying to determine the gear ratio of Option #1. Definitely 3+
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Yes, the option book only lists 3.23 and 3.55 for 72. Are you sure it's a 8 3/4?
 
AJ/FormS said:
Naw the formulas are mostly all in my head
But I do use the Wallace quite a bit for doing comparisons, on account of it is waaaaaaaaay faster, than hand-held calculators.
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Handy calculator for sure. Especially the rpm drops in each gear. (like you do).
 
toolmanmike said:
Making me wish I had a 3.09 low gearset in my 833 and a 355 or 373 rear ratio
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The only problem with doing that is dropping the rear axle ratio affects not only your final drive ratio but it will make a difference in the gear changes. Especially with the big 1-2 spit. It won’t change the RPM drop but it will change how well the engine recovers RPM.

If all you care about is a higher (ratio...lower numerically) final drive ratio than that isn’t a bad option.
 
yellow rose said:
This thread has a deceptive title. This isn’t what I thought the discussion would cover.
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YOU ARE SICK!!!!!!!!!!!!! Here's your medication:

butt2.jpg
butt2.jpg
 
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