Deciphering My Rear End

[sdcfish]

Capev -

Interesting points you make. I am lucky to have found a set of 390's and didn't realize how rare they were.

I was thinking of going with the 410 gears too, but was told that I still wouldn't be happy. I am HOPING that the 390 gears will get me comfortable at 60-65 mph so I can drive to shows out of town etc......right now I am simply crusing at 45-50mph.

Here's the money question: Can I assume that by reducing my gear ratio from a 4.57 to a 3.90, that this difference of .67 which is about 14.6% lower than the way it is now, should I expect and count on my truck now cruising comfortably at 50 mph x 14.6% better? So now my comfortable cruising speed will be 57.3 mph? I guess I could make the rear tire a bit taller which would also help....right? I really want to be able to drive on the freeway at a mellow clip at 65 which would be nice. I guess I would need to add a 5 speed transmission in there which would give me an over all ratio of .75 (overdrive 5th gear rartio) x 3.90 which is 2.92....that sounds pretty darn good, but another $3000 for a NV4500 tranny installed.

[capev86]

ummm.....you should only be saving 15% (3.90/4.57 = .85). unless you are running a 3.08 or softer, you will still want an overdrive for long trips. the combination of a strong rear gear with a good overdrive (stay away from gear vendors except on a motor home) is the way to go with a truck IMO.

i found a '66-'69 Saginaw 3 spd w/ the Borg Warner R10 overdrive unit. i have to rebuild the trans (needs a synchro) and i don't have the equipment required to operate it, so i haven't messed with it yet. i don't want to give up the strong rear gear (towing a 6,000 pound trailer with a 307-stick shift combo after all), but the .7 o/d gear will give me a 2.87 final drive and drop the rpms to 2100 at 60. that's better than a truck with straight 3.08's, plus my burb is a 3/4 ton so it has taller tires too.

if my truck had been an auto, i would have put a 200-4r in it (.67 4th gear) because it is nearly a bolt in swap for the T350 (same length). but i like the novelty of the 3 on the tree and this is my toy/toy hauler/show vehicle, not the daily driver. no automatic swap for me!

[haminator]

WELL, some new info, to me anyhow. I too prefer the look of the Eaton but have been toying with the idea of swapping for a 14 bolt for the gearing. Maybe I'll look into this parts swapping. So the 14 bolt pinion will work in the Eaton housing as well as the carrier? I have seen "1 ton" u-joints in a 3/4 ton so I assumed it would be a HO72, maybe just a bigger yoke? Also Special -K, how many coil spring Dana 60's have you seen? I've seen at least 2. One in a Chevy and one in a GMC, yes coil spring GMC. Weird huh? BTW I run my 4:10 350 Longhorn at 50-75 mph quite often but it likes 50mph the best. Longhorn man, are you sure a DRW is only 5:13? I've got 3, 2 were motorhomes, that seems a little low but I've never checked. I've driven one a little bit and it didn't seem to run higher rpms than the 4:10.

[sdcfish]

Update! New 3rd member changed out to a fully rebuilt 3.90 3rd member and it was installed right in! This full set up has a "conventional" setup instead of the "NoSlip" locker that was in there with the 4.57 gears. I was really lucky finding this setup and it was more than I bargained for getting an entire setup freshly rebuilt.

On my way home, I immediately noticed a smoothness to the rear end. I was anxious to get on the freeway and test the speed on the freeway to gage my new gear setup, but the speedometer no longer worked the same! I guess i need to find someone to help me calibrate the speedometer now??? Please help!

I have an app on my iPhone that is a speedo, so I turned it on and set it on the dash.

Pre gear change as I mentioned before was comfie at 45mph, and now the new setup showed me a comfortable 60mph. I am pretty cool with that for now, and depending on how much freeway driving I do, it might be fine.

I did notice that 3rd gear is really sweet and much longer. To me, that's where I put my foot down the most This gear change cost me $700 plus tax......not bad compared to the other options. Any help on the speedo calibration is appreciated......I live in Los Angeles area

[ltrrtl]

Sdcfish----- I have a 71 C20 Cheyney, 350 / TH350, with factory A/C. I have no idea what size gear in the rearend. After reading this thread and listening to the different performance folks are getting from the different gears. I suspect I have 4.11. I say that because it drives smooth on why at 65-70. Up Intel 6 months ago I was getting 14-16MPG. I am only getting 10-12MPG now and that is due to a fuel starvation issue that I can not figure out. Running a new edelbrock 600. All that aid, how is ur truck running and what type MPG on hwy? Also anyone have suggestion on troubleshooting fuel starvation and recent decrease in mpg?????

[Billyburban]

Originally Posted by capev86

4.11/4.56 are in more modern axles like (1/2 ton) 10 bolt, 12 bolt, and 14 bolt.

3.90's were an aftermarket gear set and therefore no longer available when it's been 40 years since this axle was used in a truck.

i swapped out the original 4.57 for 4.10 third member in my 72 Suburban C20 (307, 3 on the tree). fuel mileage improved from 10/11 to 14/15 with that 10% reduction in rpm. i also put a Q-jet on in place of the 2bbl so i'm sure the small primaries help around town. truck turns 3,000 rpm at 60 with stock rims/tires. i keep it at a steady 50mph/2500rpm on long highway trips which nets good mpg (considering) and cuts down on the racket. even once i get my saginaw/borg warner overdrive installed and have the truck sound insulated, i will still keep it at 60-65. i tend to speed only in the car.

I have a 1970 suburban 350 350turbo transmission. I was told it has 4.11 gears at 65mph rpm @ 3300 dose that sound like 4.11 ? Than I had some one tell me they were 4.10

[LockDoc]

Originally Posted by ltrrtl

Sdcfish----- I have a 71 C20 Cheyney, 350 / TH350, with factory A/C. I have no idea what size gear in the rearend. After reading this thread and listening to the different performance folks are getting from the different gears. I suspect I have 4.11. I say that because it drives smooth on why at 65-70. Up Intel 6 months ago I was getting 14-16MPG. I am only getting 10-12MPG now and that is due to a fuel starvation issue that I can not figure out. Running a new edelbrock 600. All that aid, how is ur truck running and what type MPG on hwy? Also anyone have suggestion on troubleshooting fuel starvation and recent decrease in mpg?????

He hasn't been on the board since 2/21/13 so I doubt you will hear back from him.

If your truck is running good and the engine is not screaming at 65-70 and you were getting 14-16 mpg I doubt you have 4.10/4.11 gear in it. I would say probably 3.73's or maybe even 3.08's.

I posted some charts in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Code locations on the rear housing are in posts #8 & #19.... All you need are the first two or three letters.

LockDoc

[ltrrtl]

Lockdoc,
Thanks I will check out the charts and figure out what I truly have in my truck