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Rear end gear ratio for big block

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Old 06-14-2010, 10:24 AM   #1
Red_Shift
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Default Rear end gear ratio for big block

After searching through several posts reguarding the rear gear ratios I'm a little confused as to what rear end gear I should use. The vital info follows:

1975 coupe
zz502 (5800 redline)
Turbo 400 trans (I believe 3rd gear is 1:1)
2800rpm stall converter
275/35 R18 tires (these are 25.57" diameter)
Tread width: 10.8"
I will drive this car on sunny weekends and at the drag strip on occasion.

So the zz502 puts out a lot of torque...
Click the image to open in full size.
Would this suggest a lower gear like 3.08? I currently own two gears, 3.08 and 3.90. If I install the 3.90 then I'm looking at 3587 rpm @ 70mph (seems high) and will red-line at 113mph (based on my understanding the T400 is 1:1 in 3rd gear).

That 3.90 gear is too tall right?
What gear would you run?
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Old 06-14-2010, 11:47 AM   #2
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What gear is in it now?

I have the same motor as you but a 4 speed trans and will be replacing the current 3.55 rear with a 3.08
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Old 06-14-2010, 11:57 AM   #3
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336....jmo....
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Old 06-14-2010, 12:38 PM   #4
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I run with M21, 3.08 behind a 528hp/564 ftlb 489 - with 285/40/18's it cruises 70mph close to 2600 rpm - its fine off the line too
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Old 06-14-2010, 01:11 PM   #5
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You won't like the 3.90s...the stock zz cam is done by 5200-5400 rpms and with 3.90 gears, you will be out of steam before the end of the 1/4.

I would stick with 3.36s or 3.08s since you have them. The converter will do more for you then the gears anyway.

I have a 496 and have 3.08s. It's highway friendly and the BB torque combined with a good converter pulls them fine. That being said, when I rebuild the IRS I am going to 3.36s for a little better ETs and street grunt.
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Old 06-14-2010, 04:36 PM   #6
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Thanks AJ and thanks to others who have replied.

roscobbc, your tires are 1.5 inches taller than mine. I'm not sure what the final output ratio of the M21 is. Thanks for the input.

AJ, what transmission you running?
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Old 06-14-2010, 04:57 PM   #7
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Quote:
Originally Posted by Red_Shift View Post
Thanks AJ and thanks to others who have replied.

roscobbc, your tires are 1.5 inches taller than mine. I'm not sure what the final output ratio of the M21 is. Thanks for the input.

AJ, what transmission you running?
I also have a T400, "tight" 10" converter.... At 65mph I am around 2750rpms, 70mph(gps) is 2950rpms. Converter seems to add about 200rpms or so at highway cruise. I like the 3.08s and 255/60s for highway cruising and my car still leaves pretty decent at the track (1.61 60') but if I am gonna spend the loot to rebuild the rear end and put all the goodies in there, I want to step it up just a notch....3.36 is as low as I'm going...some day when I hit the lotto and convert to a 200r4 I will be glad I up'd the gear some.

Just make sure you use a good converter, not a speed shop cheapy as they tend to slip way too much at all speeds and hurt trap speed..
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Old 06-14-2010, 05:09 PM   #8
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308 for sure.
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