Rear end dilemma...

[BK'66Step]

So I just cracked open my rear diff for the first time in preparation for cleaning it up. I discovered that it has 3.08 gears! So my end result is to have a SBC 350 with 350-400hp, 700R4 trans, and 255/60R15 tires in the rear. I was wanting the 3.73 gearing but, if I understand correctly, the lowest I can get are 3.42 gears in a 3 carrier rear end. Does anyone have a similar setup with 3.42 gears that can comment on the drivability or suggest another setup. Any other thoughts would be appreciated as well. Thanks all!

[Lugnutz65]

Originally Posted by BK'66Step

So I just cracked open my rear diff for the first time in preparation for cleaning it up. I discovered that it has 3.08 gears! So my end result is to have a SBC 350 with 350-400hp, 700R4 trans, and 255/60R15 tires in the rear. I was wanting the 3.73 gearing but, if I understand correctly, the lowest I can get are 3.42 gears in a 3 carrier rear end. Does anyone have a similar setup with 3.42 gears that can comment on the drivability or suggest another setup. Any other thoughts would be appreciated as well. Thanks all!

I use this web site to calculate RPMs at different speeds. LINK Go to the bottom section and enter the rear diff ratio (3.73 or 3.42 or whatever you have), then the tire height, the MPH (I used 70 mph), and your final tranny gear (which is 1.00 for the 3 speed factory tranny). If your tranny has OD, then you will need to figure that out. A TH700R4 final gear is 0.70)

What engine you have is not a factor except for what torque it can produce at certain RPMs and the strength of the tranny.

In the example above, a TH700R4 with a 0.70 OD gear and a 3.73 rear end and 29" tires will have your engine doing 2118 RPMs at 70MPH. That's a great number. If you went to a 3.42 rear diff (That's what my 66 parts truck had), then the engine drops down to 1942 RPMs.

I think a 3.42 rear would be about perfect for you.

I have a 0.63 OD on my manual tranny, so a 3.42 rear end will not work well for my setup.

If I had stayed with my factory 3 speed which has a final gear of 1.00, then the 3.42 would be a tempting swap. It would give me lower RPMs at highway speeds but at the sacrifice of less power in 1st gear.

You can calculate the rear diff ratio without removing the rear cover.
1. Chock the front tires and put the tranny in neutral.
2. Jack up the rear axle so both rear wheels are off the ground.
3. Mark the DS tire at the 12 o'clock position.
4. Mark the drive shaft so you can count revolutions.
5. Slowly rotate the driver's side tire and count the total tire revolutions while a helper counts exactly 20 driveshaft revolutions. (or any multiple of 10 - the more the better).
6. You should see that the driver's side tire spins and the PS stays still.
7. The tire will spin twice as many times as it normally would since both tires are off the ground.
8. The math is easy. Divide the number of shaft turns by the number of tire revolutions and then multiply by 2.
Example: 20 shaft revolutions divided by 11.75 tire revolutions = 1.70213 times 2 = 3.404. Those were the numbers I just got 2 weeks ago when I calculated mine. So it means the rear was a 3.42 rear.

Another way is to roll the truck in neutral and count the revolutions (but you don't multiply by 2 at the end). This method is kinda tough because it's harder to count shaft revolutions on a moving truck.

Anyway, I hope all this makes sense and is helpful.

[chevyrestoguy]

You might want to consider trading your 3.08 rear for a later one with 3.42s. For guys building trucks with a Th350 or a TH400, the 3.08 is the hot ticket, and they are not easy to find. I looked for a long time and had no luck, and wound up going to a 700r4/3.42 combo.

A guy around here used to grab all of the '71-'72 3.08 gear rear ends out of the local wrecking yards and would get $250 and up for them all day long. He would buy them for $125 and double his money.

[T Smith]

I'd do the 3.73's and they are available for your 3 series carrier.

[BK'66Step]

chevyrestoguy- Wow I didn't realize they were so sought after. Seems like with the TH350 you would have almost no acceleration off the line with 3.08 gears. Are you happy with your 700R4/3.42 gears?

T Smith- I thought 3.73 was a 4 series carrier. Is there a difference between the two other than that?

Lugnutz- Thanks for the info. It's good to know that either a 3.42 or a 3.73 will work fine. Was looking for good off-the-line speed, but it appears that they are fairly similar.

[T Smith]

Originally Posted by BK'66Step

chevyrestoguy- Wow I didn't realize they were so sought after. Seems like with the TH350 you would have almost no acceleration off the line with 3.08 gears. Are you happy with your 700R4/3.42 gears?

T Smith- I thought 3.73 was a 4 series carrier. Is there a difference between the two other than that?

Lugnutz- Thanks for the info. It's good to know that either a 3.42 or a 3.73 will work fine. Was looking for good off-the-line speed, but it appears that they are fairly similar.

They make them for either carrier..

http://www.summitracing.com/parts/rmg-4902801/overview/

http://www.summitracing.com/parts/rmg-4900721/overview/

[Clyde65]

I have a 3 series with 3.73's. I have the 4l60e tranny. It does great and no bogging like with the 3.08's.

I would run the 3.08's until you find what you are looking for
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[BK'66Step]

Hmm... I guess I heard wrong. Well that's good to hear then. Thanks!!

[ChevyJr]

Clyde65 - No bogging? What do you mean by that? I am looking to do this same rear end ratio switch, but am worried about mpg. running a SBC 400, 700R4, 770cfm Holley Street Avenger.

[Clyde65]

If you run ALL highway and you can run at 80 then the 3.08's will work and get you decent mileage. If you drive ANY in town then the 3.42's will work, 3.73's will be good for a mix of both and if you drive it hard and love to get her sideways, then 4 series with 4.10 are the animal you need.

I am figuring this with 28" tires but to be really sure, find your 4the gear ratio in the trans, tire size, then go to Randy's ring and pinion to see what your RPM's will be at what speed. I believe your cam specs should tell you the motors sweet spot for rpm, you want to keep the motor there to get the best economy.

Btw, if you have a 400 what kind of mpg are you thinking your gonna get?
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[Clyde65]

Ok, 350 with 400hp I miss read.

I think about 1800-2100rpm would put you in a sweet spot. I'm running about 2100 and on hwy I get about 19-19.5
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[BK'66Step]

Originally Posted by Clyde65

I think about 1800-2100rpm would put you in a sweet spot. I'm running about 2100 and on hwy I get about 19-19.5
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Yea that's why I am really wanting the 700R4. Where I live, I have to drive at freeway speeds at least for a while no matter where I go. 2700 rpms with the TH350 would just be brutal.

[Clyde65]

So what size tire are you running or gonna run?

You can do fine with the 3.08's for a while, no need to hold everything on just that.

I think you would be happier with 3.73' not much diff from 3.08 to 3.42.

My truck ran about 1800 rpm with 3.08 it now runs around 21-2200 and really likes it.

You know your NOT going to get a remarkable change in mpg right? But it will be better and you will enjoy the drive a lot more
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[BK'66Step]

I'm definitely not building this thing with MPG in mind. I'm assuming I'll be in the single digits when all is said and done... lol. I just think it would be better on the motor and I don't really want to hear the engine droning at 2800 rpms while cruising down the freeway. My plan is to have BFG Radials 255/70R15. I think it's about a 29 inch tall tire.

I realized why I originally thought the lowest gearing I could have with a 3 series carrier was 3.42. If I were to put in a limited slip carrier, that's the lowest I can go because a 4 series won't fit... now I have to decide whether I want 3.73 w/o limited slip or 3.42 w/ limited slip... or try and find a 4 series carrier rear end...

[T Smith]

Originally Posted by BK'66Step

I realized why I originally thought the lowest gearing I could have with a 3 series carrier was 3.42. If I were to put in a limited slip carrier, that's the lowest I can go because a 4 series won't fit... now I have to decide whether I want 3.73 w/o limited slip or 3.42 w/ limited slip... or try and find a 4 series carrier rear end...

??????

[BK'66Step]

Yea sorry... I'm more or less rambling/thinking out loud...

[hd74]

I have a 307 rear end. With 700r4 and it runs great. Real nice on the interstate. But I do have a 454.around town driving is fine. Maybe not for drag stripe maybe the salt flats.

[BK'66Step]

Yea not necessarily looking for a 1/4 mile truck, but I really want a good launch. I think I've settled on getting a limited slip 3 series carrier with 3.42 gears. Seeing as how I wanted limited slip anyways I think I will be happy with this setup. Thanks everyone for the input!!