(Also posted on .com)
I got a Haynes Dart and Challenger book covering 67-76 Slant Sixes (no v-8 models) from the Library. Pretty cool book, still has the worst instructions for removing upper ball joint like all other Haynes manuals though.

I was reading in the drivleine section that slant six dart and challengers w/ 7 1/4 axles came with 3:23, 3:55, 3:94 (I think 3:94 I might be off).

WTF, I thought that it also came with 2:79 as standard. I had always assumed my 225 swinger had the low ratio because it seems to have the economy version of every thing else (small u-joints, 9" drum brakes, etc...)

Is this true? I might actually have decent ration in there and cross off axle upgrade off my list? That would be great!

Thanks in advance,

Tim

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My Swinger gets the Highlight Valet Parking spot over the German sports cars on the Sunset Strip. I love my Mopar.

The book is wrong. I have a 2:76.

I guess I still have to swap the axle.

Tim

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My Swinger gets the Highlight Valet Parking spot over the German sports cars on the Sunset Strip. I love my Mopar.

3.91, 3.55, 3.23, 2.93 / 2.94 (I have seen both) and 2.76
Word is that there was also a 4.10 gear for the 7 1/4 but I have never seen one.
DD

Doc, If I remember correctly, the lowest gear for the 7 1/4 rear from the factory was the 3.91. The old "Direct Connection Program" back in the 70's, had 4.10/4.11, 4.56, 4.88, 5.13. I have the 3.91SG and 5.13 SG set ups in two different housings. I think Randys Ring & Pinion, still has some 4.10/4.11 gear sets.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Sorry Charrlie,

I definitely have a factory 2:76 Axle on my 1970 Dart Swinger, the tag says it came from the factory in Canada.

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My Swinger gets the Highlight Valet Parking spot over the German sports cars on the Sunset Strip. I love my Mopar.

I think we have a miscomunication, of terminolagy. I consider a 2.76, a high ratio, low number. You are correct, the 2.76, is a factory gear, I have one in my pile of parts. 5.13, a low ratio, high number. Probably not really the correct way of stating it.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I learn something new everyday.

Hey, educate a dummy here, when they say that 3:55 gives you better off the line, but you lose top end. What exactly am I losing on the top end? 10. 15, 20 MPH? If I want a more versatile car should I go with a 3:23?

Thanks for all your knowledge,

Tim

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My Swinger gets the Highlight Valet Parking spot over the German sports cars on the Sunset Strip. I love my Mopar.

example:

Take the redline you are comfy with, I will assume 5500 rpm

Divide that rpm by the gear ratio (this assumes a 1:1 final drive with NO slippage, slippage in an auto trans just decreases the following numbers) and that is how many rpm's the tires are making...speed will depend on tire diameter.

Making the following assumtions:
1) 5500 rpm redline
2) 28" diameter tire (7.33 feet circumference)
3) No drivetrain slippage, 1:1 final drive

The following shows max speed for a given ratio:

2.76 14607 ft/min 166 mph
2.94 13713 ft/min 156 mph
3.23 12482 ft/min 142 mph
3.55 11357 ft/min 129 mph
3.91 10311 ft/min 117 mph
4.10 9833 ft/min 112 mph

Rear end gears are a torque multiplier, the lower the ratio (higher number) the more torque you are putting to the ground, thus you can take off faster. The tradeoff is to get that additional torque to the ground, the engine must spin higher rpm for a given mph, and with a stationary redline, you run out of rpms before the greater speed of higher gears (lower numbers) is achieved.

Ummm..hope that is more clear than mud...

-S/6

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The "Silver Bullet"
87 Ram D150

Don't forget about fuel milage. Generally speaking, a lower gear (higher number) will result in worse fuel milage. A 3.23 is a good choice for a daily driver. If you do a lot of around town, stop and go driving, and not too much highway, maybe a 3.55. Don't forget, you can "tweek" the gear ratio, with tire diameter. I use a 3.91 and a 22 inch dia tire, for 1/4 mile drags. Then put on a 27 inch tall tire for street driving. That is still kind of low for interstate hiway driving, but works good for all other types. Any thing lower then 3.91 ( 4.10-5.13) should be left to race only cars, unless you have an overdrive trans.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Another way to look at rear end gearing is to think about a 15 speed bicycle. Put it in first gear and you get a quick start and can peddle-up any hill. When you get going faster, you can't peddle fast enough. Put the bike in 15th gear and you can't get moving from a standing start or carry a hill but once you get going or on downhill, the thing will haul a$$ with little effort.
Your 2.76 is 15th speed, Charlie's 5.13 are 1st gear, you need to pick something in the middle.
As stated, 3.23 is a pretty good choice as long as you don't do a bunch of freeway driving. (that gear feels good on the highway at 55-65 mph) For Slants i drive on the open road more often, I use the 2.94 ratio, it runs nice in the 65-75 mph range)
For me, I swap 8 3/4 "drop-out" rearends into many of my cars just so I can change the gear ratio.
http://www.dutra.org/pictures/parts/8.75-exp-view.jpg
The change is not as fast as flipping a lever on a bike or on an OD trans but in about 1 hour, I can completly change the way the car drives and acts buy swapping-in a different center section.
DD

I hope that helps
DD

I hope no one minds if I ask what dimension the 7 1/4 to 9 1/4" number refers to. I'm assuming it isnt useful in identifying these visually... is it the diameter of the crown gear perhaps? What are the distinguishing features of these rearends, anyway?

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

I assume the 3.23 or 3.55 is ok for around town using 904 auto?

Generally speaking the 3.55's will feel more 'fun' and motivate the car off the line better, but may kill your top end mileage, the 3.21's(3.23's if you have a 7 1/4 or 8 3/4) are a good all around gear for street/ highway use and is much more fun than 2.76's any day. Depending on the power band of you slant six's build up gear changing may not affect that much if it's not a 'radical' change. My Feather Duster's slant was put together with a lowend Comp Cam and the super six setup originally and I got 21 mpg highway with the OD gearbox and a set of 2.94's....I swapped out to a set of 3.55's and the highway number didn't change, but my light to light and highway merging was much more 'positive', also in the same vein my '74 with the stock engine with a manual 3 speed with 3.21's got 20.5 after swapping to 'Feather Duster' exhaust (2.25"), the rear developed a bad bearing and I didn't have time to pull it apart so I swapped in a set of 2.94's and gain 1 mpg highway, that rear grenaded and now I have a 2.76:1 under there...top end is great, but street acceleration isn't so 'peppy'...mileage stayed arouns 21.5 mpg highway. This summer I changed my distributor to a 'frankensteined' model with a slightly more aggressive curve than stock and it looks like I'll haul down close to 28 mpg in that 3400 tank...

When swapping out components, they have to work 'together', so if you swap to better gears and go too radical you may need to do some tranny or TQ work, later you might think the 1 barrel isn't cutting things and go to 2 or 4, then the whole cycle starts...LOL

Good luck on your gearing choice.

-D.Idiot

Doc,

Do you actually change the gearing within the rear or change the rear as unit? If the former, where do you get new gears?

Thanks.