8 3/4 gearing - 3.23 or 3.55?

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Alexhetti

Alexhetti

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I’m currently in the process of switching out the B-body 8 3/4 rear end that came with my car for an A-body one. I’m going to be putting in a sure grip 3rd member and was wondering what gearing would be best for an in-town cruiser that will occasionally see the highway. My dart has 27 inch tires, 904 trans, and a 360 with a not so mild cam... Debating between 3.23 or 3.55.
 
Alexhetti said:
I’m currently in the process of switching out the B-body 8 3/4 rear end that came with my car for an A-body one. I’m going to be putting in a sure grip 3rd member and was wondering what gearing would be best for an in-town cruiser that will occasionally see the highway. My dart has 27 inch tires, 904 trans, and a 360 with a not so mild cam... Debating between 3.23 or 3.55.
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Automatic as well... forgot the mention that.
 
Alexhetti said:
I’m currently in the process of switching out the B-body 8 3/4 rear end that came with my car for an A-body one. I’m going to be putting in a sure grip 3rd member and was wondering what gearing would be best for an in-town cruiser that will occasionally see the highway. My dart has 27 inch tires, 904 trans, and a 360 with a not so mild cam... Debating between 3.23 or 3.55.
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3.55 would be my choice between the two but play around with this program. You can see what is different by changing gears and tire diameters. Gearing Calculator | RPM and Tire Diameter to km/h - mph with Shift Tracer
 
Depends on how much you like the slow lane on the freeway? That motor will be screaming if you wanna drive 75mph with a 3.55 gear.
 
I had a 3.23 then switched to a 3.55, now I want to put the 3.23 back in after I convert it to a sure grip.
My tires are around 25.5" in height and with the 3.55 gear at 70mph i'm running 3500 rpm's.
 
Dragonsteel said:
I had a 3.23 then switched to a 3.55, now I want to put the 3.23 back in after I convert it to a sure grip.
My tires are around 25.5" in height and with the 3.55 gear at 70mph i'm running 3500 rpm's.
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Something sounds wrong there. Check out the chart in post #6
 
Around town cruising, up to 55 MPH, 3.55 gear. If you are going to run a lot of highway, I'd stay with the 3.23. Just my .02
 
forphorty said:
Sounds about right to me. You have to take into consideration torque converter slippage.
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Ya, probably a couple hundred rpm. It looks more believable when you put the right tire size. :BangHead::BangHead::BangHead:
 
I'm in a mild 340. 268 cam, headers, air gap, etc.. I have 3.23 gears with 25.5in tires... 70 mph I'm just under 3k...

I vote 3.55

I'm thinking about going up a couple sizes... I drive on the highway quite a bit but I want more tire chirping on the way to get donuts and slurpees....:D
 
DentalDart said:
I'm in a mild 340. 268 cam, headers, air gap, etc.. I have 3.23 gears with 25.5in tires... 70 mph I'm just under 3k...

I vote 3.55

I'm thinking about going up a couple sizes... I drive on the highway quite a bit but I want more tire chirping on the way to get donuts and slurpees....:D
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you'll be turning 3280 @ 70 with no convertor slip figured.
 
Let's up the gear and go 4.10 or 4.30. Who cares about highway speed. Your only going to Dairy Queen anyways. Lol
 
My 255/60-15's are 27" so a 3.55 would be fine. About 3000@70. I don't usually drive that fast on the highway any way.
 
I was running 3.55's in my Duster before I installed my T-56 and 4.30's. With the 3.55's, an 833 and 26.1" tall tires I was spinning about 3,230 at 70mph. That's definitely not "screaming", I had no problem doing that for the 30-45 minutes I was commuting. Maybe not the best gear if you wanted to drive for 6 hours straight, but it was fine for me and I use my car as a daily driver.

With a 27" tire the 3.55's won't be that bad, without any convertor losses that would be ~3,100 rpm at 70.

Also depends on what your cam is, that was part of my dilemma. My cam called for 3.91's and for good reason, I only ran the 3.55's because I needed the freeway speed. So if you have a "not so mild" cam you may want to the 3.55's to keep taking off from a stop more manageable. It will be easier for you with the auto, but still something to consider. You shouldn't have any problems running the engine at 3k rpm to maintain freeway speed, so, unless it's a long haul driver I'd probably run the 3.55's.
 
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toolmanmike said:
you'll be turning 3280 @ 70 with no convertor slip figured.
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72Dart6pack said:
Let's up the gear and go 4.10 or 4.30. Who cares about highway speed. Your only going to Dairy Queen anyways. Lol
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toolmanmike said:
That'll get you there quick.
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Well... Im sold...

I do drive to school at least 1x a week and its about a 20 minute drive from my house majority of the drive on the freeway

Converter slip? I guess I better get a good converter next time right. Lol.
 
toolmanmike said:
My 255/60-15's are 27" so a 3.55 would be fine. About 3000@70. I don't usually drive that fast on the highway any way.
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This is the same tire and wheel setup my dart is using. Speed on the highway really isn’t a priority and I’m going to be avoiding the highway as much as possible until I get the steering tightened up a good bit/rebuild the frontend. This will mostly be street cruiser to go into town and tear up some backroads lol
 
Lots of intown with occasional highway...
I got to say that again for the people who don't listen LOL...
3.55...
 
j par said:
Lots of intown with occasional highway...
I got to say that again for the people who don't listen LOL...
3.55...
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j par said:
Lots of intown with occasional highway...
I got to say that again for the people who don't listen LOL...
3.55...
Click to expand...
Everyone’s got me pretty sold on the 3.55. I think it’ll be a sweet ride once I get them in there. Definitely won’t mind the extra kick from stop and go’s
 
That kindof depends on the not-so-mild cam ....... and your elevation.
There is no point in trying to cruise a big cam at low rpm, in terms of fuel economy.
At or near sealevel; I'd gear a city car to hit 60mph in Second gear(A904) at about 200/300rpm after the power peak. With a 5000 peak, I'd be thinking 4.10s; for 60mph=4430 @zero-slip/say 5100 on the tach@15% slip.
But I wouldn't be taking it far on the Freeway on account of 65=3310 by the math/ say 3480 on the tach@5% slip.....

Between the two offered gears, I guess 3.23s. Why? because 50mph= around 5600 on the tach at the top of first gear, whereas 3.55s will be hitting 6150. If it's slow off the line, that's what they make hi-stalls for. If it spins 3.23s thru first, it won't be any quicker on the street with 3.55s.

But as the elevation goes up, and the engine loses power, sooner or later I'd be wanting the 3.55s.
 
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