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Discussion Starter · #1 ·
Hey guys...I am trying to decide on what gear I need to swap to in my 68. I have a 402 BBC with a Turbo 400 and what looks to be a 2:73 gear. I am sick and tired of one-wheel-peels and would love to feel the rear go sideways!! So...I am going posi.

Now, I figure by best option would be a 3:73 with a OD tranny like the 700R4. I really don't have a ton of extra cash to get a new OD tranny but if i have to...then I will jhave to wait a while.

The other option I have considered is a 3:42 behind my existing Turbo 400. Does anyone have this setup? Would this be plenty or should I just hold off and do 3:73 with OD in a few months.

I don't race but I want to feel the car 'twist' if you know what I mean. I do cruise to some shows about 65 mph (70mph tops).

(p.s. my brother can get me any gear parts that I want for nothing so I want to make sure i make the right choice instead of burning my 'free' pass on the a gear I won't be happy with.)

Thanks for any input!
 

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Hey BBG , I cant recommend a gear but I can tell you from experience that you don't want to be stuck with a really low gear. I'm running a 4:11 myself and it ain't no fun cruising at 60 or 65. But she will get sideways off the line and its lots of fun from 0 to 60. Thats all I got. ;)
 

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My 71 formula with a 455 and 3.42 would destroy the 265-15 tires with posi. I think it's a perfect gear, and then the addition of overdrive, sweet mileage.
Dale
 

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Michelle -

Which rear do you have? The reason I ask is not all ratios are available for all rear diffs. I will try to go back through some of your other posts and see what engine specs you have, convertor stall speed, and what diameter rear tires you're running. All of that comes into play to some degree.

I have a 402 with a very mild cam (a Crane Resto grind that is the 396/360HP equivalent) and a TKO600 (2.87 1st), and an 8.5-inch 10-bolt with 3.42. When I fired it up last week, anything over half throttle in 1st or 2nd just blows the rear tires off. Some of that is the small tires I have on now, but if I can't hook it up on a "normal" size tire like a 255 60 15, I will actually be changing to a "higher" numerically lower set of gears.

Your TH400 has a 2.48 1st gear, so a 3.42 or even 3.31 or 3.23 would be fine. If you want to really be able to cruise and plan to do the 700R4 down the road, even a 3.08 or therebouts is good (700R4 has a 3.06 1st gear).

Now if you're running a tall, wide rear tire, you may want the 3.42 or so to get things moving.
 

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Discussion Starter · #9 ·
Ok...sorry for not giving more info...I have the following:

402 BBC .30 over wtih mild cam (.280/.520)...etc...
TH400
TCI Stall Saturday Night Special (i think up to 2000 stall...)
2:72 10 bolt
Rear tires = 245/60/15 (26.57")

I think you should be able to see what the car looks like on my picture attached to my profile.

Since I posted last night, I also ran my specs through an RPM calculator and found that the RPM at 65 MPH is only 270 RPM lower than the 3:73. So, now I guess either one would need a 700R to cruise at 65. Doest that sound right?

BTW, Here is the site I used: http://www.differentials.com/calc.html

Thanks for all you input!!!
-Michelle
 

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Nice car. And you like hunting & fishing too !!

I would stay with 3.42 gears for street use. You should be able to get sideways with your 402. I run a fairly mild 427 with 3.31 gears and M20 and would not want lower gears for highway use.

Good luck.
 

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Discussion Starter · #13 ·
Nice car. And you like hunting & fishing too !!

I would stay with 3.42 gears for street use. You should be able to get sideways with your 402. I run a fairly mild 427 with 3.31 gears and M20 and would not want lower gears for highway use.

Good luck.
Yeah, I am a big red neck...but it is a lot more fun that sitting around painting my nails! O, wait..i don't hav eany nails...
 

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Ok...sorry for not giving more info...I have the following:
You're excused.

I think you should be able to see what the car looks like on my picture attached to my profile.
Very nice ride.....nice rock on the finger also!

Since I posted last night, I also ran my specs through an RPM calculator and found that the RPM at 65 MPH is only 270 RPM lower than the 3:73. So, now I guess either one would need a 700R to cruise at 65. Doest that sound right?
Strange, I thought of the same thoughts this AM, but was called to a meeting before posting.

Thanks for all you input!!!
-Michelle
You're welcome. My thoughts are these, you only want to do this gear change once, because of cost and time. If you are planning on swapping to an OD trans later, I'd go with a 3.73 because of the final engine rpm. With OD and a 3.73 ratio, this will put the engine into a more efficient rpm by placing the engine into the main fuel circuit and get it out of the off idle circuit. I'm sure I'll get comments on this.

If not anything else, it will be better music to your ears with the windows rolled down. Besides, it sounds better when you tell others you have a 3.73 than it does when you say 3.42. Try it in front of the mirror, you'll see what I mean.

But then, I'm not a professional advisor either.
 

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I like this calculator because it lets you see different RPM values for different gear sets at the same time. It is a little hard to look at though.

http://www.angelfire.com/fl/procrastination/rear.html

If you have the original 8.2-inch 10-bolt, I am fairly certain 3.42 is not an option for you. It's 3.08, 3.36, 3.55, or 3.73 in off the shelf R&P for the 8.2.

And, as you can see by the varying opinions, there are a bunch of things to think about, and it is all about making it all work together. You really have to sit down and run through the calcs for each ratio in each trans gear, and compare that to what makes your engine happy.
 

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Discussion Starter · #17 ·
You're excused.

If you are planning on swapping to an OD trans later, I'd go with a 3.73 because of the final engine rpm. With OD and a 3.73 ratio, this will put the engine into a more efficient rpm by placing the engine into the main fuel circuit and get it out of the off idle circuit. I'm sure I'll get comments on this.

.
My brother said almost the same thing. His comment was that the motor would run a LOT better with a 3:73.
 

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Discussion Starter · #18 ·
If you have the original 8.2-inch 10-bolt, I am fairly certain 3.42 is not an option for you. It's 3.08, 3.36, 3.55, or 3.73 in off the shelf R&P for the 8.2.

QUOTE]

The local driveline shop told my brother that i coudl go to a 3:42 and use the existing carrier or spool or whatever it is. I will double check to make sure.
 

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The local driveline shop told my brother that i coudl go to a 3:42 and use the existing carrier or spool or whatever it is. I will double check to make sure.
I don't think so, 3.42:1 is a '70 & above, read 8.5 inch diameter, gear ratio where 3.55:1 is a '67-'69 ratio, 8.2 inch diameter.

At least for any OE gear ratios, aftermarket can produce anything to fit everything.
 
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