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school me on rear end gearz #447389
08/23/09 06:43 PM
08/23/09 06:43 PM
Joined: Jan 2003
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Shelby, Ohio
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2abodymcodes Offline OP
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69 roadrunner with the original 383 and 727 tranny. for driving around town and short trips which would be the better set of gearz for my 8 3/4 ? 3.91 or 3.55 i have one of each and don't know which to use. i never race and will not be taking it on any long trips. your opinion please, and why.

Re: school me on rear end gearz [Re: 2abodymcodes] #447390
08/23/09 07:00 PM
08/23/09 07:00 PM
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Mass
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If you want a stoplight terror go with the 3:91,....with a 27" tire your best speed on the highway will be about 60MPH


The 3:55 are also good gears, a little dogged though in comparison to the 3:91, the 3:55 is good on the highway up to about 100MPH, without really pushing the motor, not bad for around town


Mike

Re: school me on rear end gearz [Re: 2abodymcodes] #447391
08/23/09 07:12 PM
08/23/09 07:12 PM
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Kirkland, Washington
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Using Randy's Ring & Pinion calculator and a 27" tire I get 66 mph with the 3.55 at 3000rpm and 60 mph at 3000 with the 3.91. Auto tranny.

With typical tire sizes the 3.55 really is the compromise gear, doing both tasks adequately but not optimally. The 3.91 are far better around town (for performance driving) but they do poorly on the highway on long trips.

Re: school me on rear end gearz [Re: DAYCLONA] #447392
08/23/09 07:12 PM
08/23/09 07:12 PM
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Shelby, Ohio
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2abodymcodes Offline OP
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thank you. that's what i needed to know.

Re: school me on rear end gearz [Re: 2abodymcodes] #447393
08/23/09 07:21 PM
08/23/09 07:21 PM
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JohnRR Offline
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becareful switching to 3.55 or 3.91 , make sure your cooling system is up to task , Mopar put 26" radiators in cars with both those ratios for a reason .

me I'd go with 3.91's

Re: school me on rear end gearz [Re: JohnRR] #447394
08/23/09 07:47 PM
08/23/09 07:47 PM
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S.E. Conn
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I recomend you go with the 3:55's.

Why?

So you can sell me your 3:91's.


2005 300C Hemi
2014 Ram ProMaster 2500

I Rekey, Repin old Chrysler locks and cut keys by code
Re: school me on rear end gearz [Re: JohnRR] #447395
08/23/09 07:50 PM
08/23/09 07:50 PM
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Upper Midwest
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Just a rule of thumb that we used to go somewhat by and will get you in the ballpark anyway: With 3.91 gear and a standard size tire IE 26/27 double the first two number of the tach reading. 3000 = 60 MPH, 3500 = 70 and so on. Not super accurate but will get you in a close ball park.

Re: school me on rear end gearz [Re: Pacnorthcuda] #447396
08/23/09 08:38 PM
08/23/09 08:38 PM
Joined: Dec 2007
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Quote:

Using Randy's Ring & Pinion calculator and a 27" tire I get 66 mph with the 3.55 at 3000rpm and 60 mph at 3000 with the 3.91. Auto tranny.

With typical tire sizes the 3.55 really is the compromise gear, doing both tasks adequately but not optimally. The 3.91 are far better around town (for performance driving) but they do poorly on the highway on long trips.








The speed calculators are a good base reference, but are general, and can't compensate for real world conditions, such as air drag, HP, aerodynamics of the car, smblk, BBlk, etc,.......my Challenger T/A with 3:91's with a 27 " tall tire, would do about 60 MPH at about 4000 RPM,......my 70 SuperBee with 3:91's with a 27 " tire, and 440 would do about 60 MPH at about 3800 RPM,.....neither was enjoyable on the highway,.......that's what a Keisler is for!

Re: school me on rear end gearz [Re: DAYCLONA] #447397
08/23/09 09:11 PM
08/23/09 09:11 PM
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Grand Prairie,Texas
stumpy Offline
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Play with this and see what you get. http://vexer.com/68rt/speed.html

Re: school me on rear end gearz [Re: stumpy] #447398
08/23/09 09:17 PM
08/23/09 09:17 PM
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Upper Midwest
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Quote:

but are general, and can't compensate for real world conditions, such as air drag, HP, aerodynamics of the car, smblk, BBlk, etc,.......my Challenger T/A with 3:91's with a 27 " tall tire, would do about 60 MPH at about 4000 RPM,......my 70 SuperBee with 3:91's with a 27 " tire, and 440 would do about 60 MPH at about 3800 RPM,...


No way are you going to turn that kind of RPM's with those gears and tires and no way would there be a difference with the same size tires and gears. even with a 4.88 gear at 4000 you should be in the area of 65 MPH with a 27 inch tire. With the same height tire and rear end gear ratio the RPM's would be the same no matter what the conditions, engine size, aero of the car or truck etc. Engine RPM per gear ratio and tire size will not change as long as both transmissions have a 1-1 out put drive which most did unless OD equiped which was not an option on those cars.

Re: school me on rear end gearz [Re: MoparforLife] #447399
08/23/09 09:39 PM
08/23/09 09:39 PM
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oklahoma
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forphorty Offline
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Torque convertor slippage must be taken into account.

Re: school me on rear end gearz [Re: forphorty] #447400
08/23/09 10:01 PM
08/23/09 10:01 PM
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Quote:

Torque convertor slippage must be taken into account.


Let the good Lord help you if you have that much converter or transmission slippage. Either one would be toast.

Re: school me on rear end gearz [Re: MoparforLife] #447401
08/23/09 11:17 PM
08/23/09 11:17 PM
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oklahoma
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forphorty Offline
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Quote:

Quote:

Torque convertor slippage must be taken into account.


Let the good Lord help you if you have that much converter or transmission slippage. Either one would be toast.


I guess the auto manufacturers were wasting their time when they went to the trouble of designing lockup convertors. I do agree with you that Dayclonas rpm seems high for 60mph. From my own personal experiance, going from a 11 inch 2500 stall, to a 10in 3500, to a 9.5 in 4500,every one has run a little more rpm on the highway than the last.yet after years of driving and thousands of miles, my tranny is not toast. I do have before me an excerpt from the july 95 issue of Hot Rod. In it a nova with 3.00 gears had its 11 inch convertor swapped for a 10 inch.The rpm cruising at 60 mph rose from 2600 to 2950. With the rearend gears changed to 3.25, rpm at 60 was 2900 with the 11in and 3100 with the 10 in.

Re: school me on rear end gearz [Re: forphorty] #447402
08/24/09 06:30 AM
08/24/09 06:30 AM
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Upper Midwest
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Quote:

Quote:

Quote:

Torque convertor slippage must be taken into account.


Let the good Lord help you if you have that much converter or transmission slippage. Either one would be toast.


I guess the auto manufacturers were wasting their time when they went to the trouble of designing lockup convertors. I do agree with you that Dayclonas rpm seems high for 60mph. From my own personal experiance, going from a 11 inch 2500 stall, to a 10in 3500, to a 9.5 in 4500,every one has run a little more rpm on the highway than the last.yet after years of driving and thousands of miles, my tranny is not toast. I do have before me an excerpt from the july 95 issue of Hot Rod. In it a nova with 3.00 gears had its 11 inch convertor swapped for a 10 inch.The rpm cruising at 60 mph rose from 2600 to 2950. With the rearend gears changed to 3.25, rpm at 60 was 2900 with the 11in and 3100 with the 10 in.


No not at all but the slipage warrented for this was very abnormal. Or a bad tachometer. There is no way to turn either one of the RPM's listed with the speeds listed.


Clean it, if it's Dirty. Oil it, if it Squeaks. But: Don't fix it, if it Works!
Re: school me on rear end gearz [Re: Pacnorthcuda] #447403
08/24/09 09:27 AM
08/24/09 09:27 AM
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Florida
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I'm in a similar situation trying to decide between 3.55 & 3.91. I DO plan on driving the car long trips on the interstate though, which leans me towards the 3.55, but I will also be installing a Gear Vendors overdrive, which leans me towards the 3.91.

Which one should I pick?

Tav

Re: school me on rear end gearz [Re: cataclysm80] #447404
08/24/09 09:55 AM
08/24/09 09:55 AM
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If your talking about an 8.75 rear, then get multi rears.

I currently have a 3.23 (factory original)in my RR

If I want to go on a long trip I pop in my 2.76. If I want to toast the tires I pop in my 3.91.

Granted I spent more $$ on the 3.91 being rebuilt and it's a sure grip. Didn't need any sure grip for the 2.76, it's open.

1/2 hour to change pigs. I don't do it every week but how many times do you go on long trip?

Just my It's an option.

If not, go with the 3.55 as said before, an all around compromise.

Frank


Remember - 2 wrongs don't make a right - but 3 rights make a left!
Re: school me on rear end gearz [Re: cataclysm80] #447405
08/24/09 12:01 PM
08/24/09 12:01 PM
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U.S.S.A.
JohnRR Offline
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Quote:

I'm in a similar situation trying to decide between 3.55 & 3.91. I DO plan on driving the car long trips on the interstate though, which leans me towards the 3.55, but I will also be installing a Gear Vendors overdrive, which leans me towards the 3.91.

Which one should I pick?

Tav




gear vendors , definitely the 3.91







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