Need more rearend help and advice...please

[lts70]

Well My newest project keeps getting bigger and bigger. I started out buying some rims for Christmas. I decided to get 5 lug because that was the only way to get this type of wheel. Then I bought 5 lug hardened rear axles from summit racing to swap out my 6 lugs (70 Chevy W/ 6 lug disc brake kit, I already have 5lug front rotors also). Then I decided while I am in there I may as well change out my seals and bearings and put some synthetic oil and posi additive. http://67-72chevytrucks.com/vboard/s...d.php?t=318405

Then I asked around to see what gear ratio I had. My truck is a 1970 Chevy C-10 SWB that from the factory had a 307 engine, a 4 speed with granny low and a posi rear-end. So I was told because of these options it had a 3.07 posi rear-end. Not exactly the best ratio to go with my NV3500 apparently. I think the ratios for the nv3500 are 1st-4.02 / 2nd- 2.32 / 3rd- 1.40 / 4th- 1.00 / and 5th- 0.73 (correct me if I am wrong). So now I have decided that while I am in there I might as well put new gears in it. Hoping that the lower gear ratio will wake it up in lower gears and make my OD more usable.

It does good but the last truck I had this built 327 motor in ran like a whipped dog, Before it was in a 1968 SWB GMC heavy half ton from the factory with the 6 lug in front and 8 lug in back. I don’t know what ratio the 68 would have had. Now it just does alright in my 70 Chevy in comparison what it did in the 68.

The tires I am running at the moment are / Backs 285 70 R15 - Fronts 255 70 R15. If I do go to a different tranny in the future it will be the 4l80E with a 6.0l in front of it.
Please let me know what you guys would do.

Should I just go through the entire rear-end or would that be a waste in comparison to buying a used axle with the correct stuff in it.

What rear-end ratio would you use with my set up and that would go with my possible future upgrade?

Thanks a ton in advance guys

[lts70]

Here is some good information I got from kcjones.

"Keep your posi and the 5 lug axles in the 70 rear end, you can simply buy new gears for your 3 series carrier - they make new ring and pinions now for almost any ratio that are either series 3 or 4. Your carrier is definitely a 3 if you have factory 3.07 gears.

Or, buy quality used 3.42 ring and pinion from virtually any truck 12 bolt, and they'll bolt right in - 3.42 and numerically lower gears were series 3 carrier design from the factory. If you find used 3.73 or numerically higher gears, they'll be the series 4 - so in that case you'd have to change your differential because a series 3 diff carrier won't bolt to a series 4 ring gear.

You'll just need to take it to a rear end shop to have them set the backlash and pinion spacing for you (should be less than $100 to do that), or you'll rip apart those new gears in no time!

You can run 3.73's, 4.11's, whatever you want, your rear end housing is just like all truck 12 bolt housings. However, if you want to run anything numerically higher than 3.42, you need to make sure it fits Series 3 carrier (which is the series of your differential carrier). The factory made 3.42 and num. lower gears Series 3 carrier, and 3.73 and num higher Series 4. So, if you buy new, buy any ratio you want, just make sure it's Series 3 so you don't have to also swap out your Posi differential. If you get used factory gears, the only ones that'll fit are the 3.42's and num lower."

I have 3.42's with my 29" tall rear tires, and I run roughly 2,900 rpm at 65. Pretty good compromise between 3.73 and 3.07."


Is it possible to find out for sure if my rear end is a 3.07? I thought I remembered some one saying you could find numbers stamped on them some where that say for sure. Does any one know where these are?

What would you guys go with 3.42 or 3.73?

Here is a link to summits Gear Ring and Pinion Sets

http://store.summitracing.com/egnsea...700+400137+115

Is there anyone that can help me out with my gear ratio? Or know a site that explains or has a gear ratio calculator? I understand the princibles but don't know exactly what I need for my rearend gear ratio to go with my tranny. All I know is what I have now is too high.

[DavidB]

I say 3.73 for sure since you have an overdrive. I have run this gear in several cars (with a .68 overdrive) and was very happy with it. Lots of low end and you can still cruise on the interstate pretty decent.

[Boog]

You won't see or feel much difference between the 3.07 to 3.42 change. With the OD you should probably step up to the 3.73 for best all around gear. It will pull great in comparison and the OD will help on the highway rpm.
If you would care to play with some combinations try these gear calcs.
.
http://www.richmondgear.com/streetcalculators.html or
http://www.ringpinion.com/Calc_RPM.aspx

[lts70]

Thanks a ton guys, I was beginning to think no one was going to respond.I will be leaving you positive reputation points on your User CP. I was thinking 3.73 but didn't know for sure, if 3.42 is too low would 4.11 be to high?

I just got out from under my truck looking for codes on the axle. This is what I found. On top of the passenger tube there was a TPF12 30W2 number with a cock eyed E to the cab side of the 12. Looking from the back towards the cover plate there was 4 sets of numbers on the driver side the top one was CON1 upside down, second 1234, third 8788, and fourth L188. on the driver side opposite side looking back there was a large GM 52. Does any one know if these can tell be what I have?

Acccording to this site I found I have a 3.73. http://www.americanclassic.com/paint.htm?p1=axles

That doesn't seem right to me with how my OD performs. Looks like I need to pull the cover and count some teeth. Does anyone have a chart that tells you what ratio it is by how many teeth are on the ring gear or is it stamped on it somewhere?

[WorkinLonghorn]

If you have a posi just raise the rear and count how many revs the driveshaft goes for one tire rev. Or if it is open,you have to multiply by 2 with the opposite tire on the ground.
I saw a method once that had you do the math for 10 revs of the wheel for accuracy sake but the difference between a 3.07 and a 3.73 should be easily apparent.

[lts70]

Thanks WorkinLonghorn, I will do that tonight. I did not want to open her up till I was ready to do everything, this way will let me still do that.

[lts70]

I did the tire and drive line count and it was 3-3/4 turns on the drive line equals 1 turn on the tire. Does that mean I have a 3.73 gears?

[big_al_71]

I would say that your truck does have a 3:73 gears because my 71 had the same exact set up 307 manual trans and when I re did the rear end it had 3:73 indeed.

[WorkinLonghorn]

I did the tire and drive line count and it was 3-3/4 turns on the drive line equals 1 turn on the tire. Does that mean I have a 3.73 gears?

That's it!

[lts70]

Thanks WorkinLonghorn. If I went will 4.11 would it wake it up noticeably from 3.73? Would 4.11 not be good with my OD?

[haysonj]

Originally Posted by lts70

Thanks WorkinLonghorn. If I went will 4.11 would it wake it up noticeably from 3.73? Would 4.11 not be good with my OD?

I have 4.09 gears with a T400 and gear vendors OD It has great low end power and cruises 65 at around 2200. I do have 35 inch tires though so yours would run at a higher RPM

[SHORTBED HARDHEAD]

I hope someone can help me. I have a 1969 c-10 and 1 want to swap out the axle from 6 to 5 lug. Is this possible without changing the whole housing? I measured the one that is on my truck now and compared it to a 1970 and the 69 is about 3/4 of an inch shorter on each side than the 70. Any suggestions or help would be appreciated.


Thanks,

shortbed hardhead

[lts70]

Looks like the 3.73 I have in her are just what my truck needs for the type of use I use her for. Thanks for all of the help guys. I got my truck switched over to 5 lug on all four corners last night. Just a couple little things and I will be rolling on my new rims.