how to identify rear ends..

[oldblue1968chevy]

id like to know what my rear end gear ratio is, is there a marking somewhere to tell,and what rearends are better for what

i have 3speed on the column and 350ci motor

*what will give me better gas mileage
what will give me better trailer pulling power
*what will give me more off the start pull you in the seat lol

hope this make sense

the * ones are the ones id really like with gas mileage 1st probably

[68gmsee]

I depends on the type of rear axle. On most GM's the number is stamped on top of the passenger side axle. Probably covered with grime so you have to sand and clean then go to GMCPauls site and check it.

http://www.gmcpauls.com/vin_model_engine_axle_info.htm


Re: mileage, performance, off the line, etc.. I'd start by first determining what you have. But, you might be asking too much for just one rear axle. You could probably compromise but the combination of rear end axle ratio, engine power and transmission are all going to dictate what you end up with.

[oldblue1968chevy]

ok i look wen i get home thans

[VanceH]

Hey this is a great website!! Is this guy a member www.gmcpauls.com? Has anyone purchased things from him??

Thanks

[68gmsee]

Yeah, GMCpauls's a member. I've purchased some items from him.

oldblue1968chevy, if you're looking for better gas mileage, I'd look for one that has the "higher" gear ratio. In other words 3:08 to 1. A "lower" gear ration, say 4:11 to 1 would give better acceleralion but at the expense of gas mileage on the highway.

The reason I hyphenated the "higher" and "lower" is because it's sort of confusing.

From an Olds site..http://www.classicoldsmobile.com/tech/69.shtml

The terms higher and lower can get a little confusing when refering to rearend ratios. If someone changes to a higher gear ratio they are actually changing to a numerically lower ratio. If you were to swap a 2.93:1 gearset with a 3.42:1 gearset you would be switching to a lower gear even though the ratio of driveshaft turns to turns of the wheel is higher. If you swap a 3.42:1 with a 2.93:1 you are switching to a higher ratio. I don't how high ended up meaning low numerically or low ended up meaning high numerically, but they do.

[VanceH]

First let me say that me and gmc paul are going to be best buds!!!

I never really understand the whole higher lower gear thing, what if you wanted to cruise 80 with low rpm's and you had a 4.11 set up would you go lower or higher?

[70k52wd]

Originally Posted by VanceH

First let me say that me and gmc paul are going to be best buds!!!

I never really understand the whole higher lower gear thing, what if you wanted to cruise 80 with low rpm's and you had a 4.11 set up would you go lower or higher?

Your going the wrong way. 80mph+4.11=pegged tach
80mph+3.08= less off line pull, but better cruze ability and much lower rpm's at 80mph. Depends on the weight of what your pulling as to what ring gear to choose. If your pulling say a 3000lbs bass boat, go with a 3.42 ring gear. Good pull from start, O.K. gas mileage. And you can cruze at 80mph all day long. Hope this helps.

[Critter]

To cruise better you need a "higher" ratio aka 3.73, 3.08, etc. But keep in mind that is a numerically LOWER. Real confusing isn't it.

[VanceH]

cool i will never be pulling anything with this truck but my fat butt!! so i think i am going to go with the 3.08 and the new crate 350 backed up with a th400 should be set for cruising!!

[68gmsee]

Originally Posted by VanceH

cool i will never be pulling anything with this truck but my fat butt!! so i think i am going to go with the 3.08 and the new crate 350 backed up with a th400 should be set for cruising!!

Good selection! But gas mileage will still be 12-14 mpg on the highway.
Maybe a 700r4?

[PanelDeland]

GMC Paul is a member and a BB vendor.There are several vendors who frequent the site.Most of them I have met or dealt with and all are stand up guys.Most are also builders of these trucks and tend to sell the best quality parts available since they use them also.Some have "real" jobs but sell parts also so may be a bit harder to get in touch with though I haven't heard of any problems.

[Shane]

chances are with an I-6 and a manual 3 speed that you're rear gear is a 3.73

[oldblue1968chevy]

okay well i scraped .5" grease off the rearend for 5 10 minutes saw no marks or nothing..im looking on pass. side on the top of the axle, is this correct? thanks

geez i didnt think it was that dirt under there

[LOW DOUGH]

Actually if it is a 3 on the tree it is probably 4.11. It is my understanding that most came this way. It is also my understanding that most automatic trucks came with 3.73. I am referring to half tons here. This is how my 2 truck are. The 69 is an auto w/ 3.73 and the 71 was a 3 on tree and has 4.11.

[eightbanger]

I'll have a look for that ID on mine also....let you know if I find it.

[oldblue1968chevy]

thanks fellas..its awful tight under there...okay, well whats a 4.11 rear end good for..

when yall find it, theres a good'n rusty swb 5 lug trailer accross the street in a field..chance i could have it free, got 2 five lug wheels and .5 of a frame, and a VERY rusty bed idk wat the rearend would be in it...but if its worth it id swap it to mine and have a 2nd spare rusty bed

thanks for whoever locates were on the axle the numbers are stamped!

[too much stuff]

Originally Posted by LOW DOUGH

Actually if it is a 3 on the tree it is probably 4.11. It is my understanding that most came this way. It is also my understanding that most automatic trucks came with 3.73. I am referring to half tons here. This is how my 2 truck are. The 69 is an auto w/ 3.73 and the 71 was a 3 on tree and has 4.11.

I don't think i have seen any 4:11 ratios in a half ton. I know they were available, but not common. Most that i have seen are 3:08. I think that was standard. I have seen 3:54 gears in half ton gmc trucks. 3:73 was more common, but still an option i believe.
You could always jack the truck up and count the revolutions from the tire to drive shaft ratio...

[oldblue1968chevy]

ok..thats what dads friend said but i was like f that easy to just read some numbers....

i didnt realize they had 3" of grease ontop of them, and that stufs baked on lol

tthank!

[Sinisterspeed]

Easiest way I have found to figure it out for sure. Is jack up the rear from the center. Get a buddy to do the other side. Set both rear valve stems at 6 o'clock rotate both tires 1 full turn while counting how many turns the driveline does.
just over 4 full turns. 4.10 or 4.11
just under 4 full turns 3.73
3.5 turns 3.42 or 3.54
a hair over 3 turns 3.08

Lower the number better gas mileage/less take off power.
higher the number better take off power/less gas mileage.

This is why alot of people like using the 700R4 trans dispite it not being as strong as a TH350 or TH400 in stock form anyway.

The 700R4 has a lower first gear and a taller 4th gear thus you can get better take off power while using a taller gear ratio (lower number) and getting that gas mileage you were talking about.

This is exactly what I plan on doing with my 57
400 sbc (already in there) or it would have been
600 cfm carb
700R4 trans
3.08 or 3.42 gears. Not 100% sure on that yet.

Hope this helps explain the what and why

[oldblue1968chevy]

thanks, too bad i dont have a buddy on hand and its dark out..but will try to do that this weekend or tom. after school..thanks!

[ERASER5]

Block the front wheels, jack up one rear wheel, put the truck in nutral. Draw a line on the drive shaft so you can see it. Rotate the tire 2 times while counting the revolutions of the drive shaft. 3 turn = 3.08, 3.5 turns = 3.54, 3.75 turns = 3.75 and so on. There are only a few actual gear sets so a purt'near readinf on the drive shaft is good enough. Hope this helps.

Oh yea, the 2X wheel revolution is due the the open differential spider gears taking up 1/2 of the movement. If you by chance have a limited slip rear end, lack up both tires and only rotate the tires once.

[oldblue1968chevy]

mines limited slip, i can only do 1wheel peels lol

[thetruckrman]

Hey fellow truck lovers. I'm new to the group, and I have found some great info on here. Thanks.

So, I thought I would revive this good thread with some info on identifying our rearends.....

http://www.chevydiy.com/history-and-...differentials/