Gear ratio

[SS Tim]

A 72 C20 with a 350/TH trans would have 4.10 gears. A coil spring HO52 rear was also offered in an optional 4.57 ratio. The leaf rear Dana 60 was not offered in optional ratios in this application. As marked the left column shows the standard ratio and the right shows any optional ratios.

[Scott>72-GMC]

Thanks that narrows it down I have a coil spring rear so ill look into it when I get it here. Thanks again
Posted via Mobile Device

[eagleuh1]

Mine's a leaf spring C20 with Dana 60. I swapped the whole rear end out, housing and all. The 3.54:1 are much better on the highway. Even towing the airstream I like them. Good luck with your hunt for the right gears for you.Jim

[Mustard72]

Cool chart! I thought mine was an original 350/3OTT C20, though, and that isn't shown on the chart. It only shows those engines with TH automatics. I'll have to do some more digging on my truck. Maybe it didn't have a 350 to begin with (although it has the 350 emblems). Or, maybe it's a super rare 350/3OTT worth half a million dollars

[SS Tim]

Originally Posted by Mustard72

Cool chart! I thought mine was an original 350/3OTT C20, though, and that isn't shown on the chart. It only shows those engines with TH automatics. I'll have to do some more digging on my truck. Maybe it didn't have a 350 to begin with (although it has the 350 emblems). Or, maybe it's a super rare 350/3OTT worth half a million dollars

You are not looking at the chart correctly.
The engines on the left 292,307 and 350 share the same transmission and final drive charts. So a 350 would have a column shift 3-speed as standard with both a 4-speed and the Turbo-Hydramatic as options.

[eagleuh1]

There goes the half a million from auctioning it off on Barret Jackson. LOL.

[Mustard72]

Huh. I wonder if I can still return the small carribeann island I bought with the money I was going to make...

Not the first time I've looked at a chart wrong

[SS Tim]

Had to do a few total re-edit "saves" after reading "x" and typing "y" myself. Usually with about 2 min. left of my edit window!

[KEL]

Originally Posted by SS Tim

Since there are questions, here is the 72 Chevrolet powertrain chart. A GMC should be the same. A Chevrolet 5200 is an Eaton HO52 and Dana 5500 is a D60.

Hey Tim !
What book is that in ?
I can't find it in any of mine.
Thanks
Kel

[SS Tim]

It is right out of the 72 GM truck data book. Mostly I use the download copies to reduce wear on my paper, plus it easier to paste from. Go to GM Heritage and download everything from 67-72 including blazers and suburbans. There is also a parts manual online too. These links are repeatedly in previous posts by me. Right now I am in the middle of trying to resurrect my dog misters. Having a heat wave and near 50* shifts daily. Gotta go.

[KEL]

Thanks
I have downloaded most of them.
Must have skipped right by that one.
Thanks again

[69vetteside]

was about to post this same question.My 69 was originally a 307/muncie 4 speed. what ratio would that have been out back.
it now has a 350/t400 and sits on 2100 rpm at 55mph.
I checked the old way with lifting the wheel and checking rotation and got 3 3/4 turns of drive shaft so would it have had a 3.73 as original for the 307 engine combo.
I have scored another 12 bolt diff from a mate that has 4.11 eaton posi and has been shortened 2" for nothing when he swapped to a 9"

[SS Tim]

Don't have a 69 ratio chart in front of me but 3.73 was the base ratio for a 69 C10.
In 70 a 307 V-8 with a wide ratio four speed CH465 would have 3.07 stock with 3.73 and 4.11 optional. However a 307 V-8 with the close ratio NP 435CR would have 3.73 stock with 3.07 and 4.11 optional.
If you want to post your axle code we can look that up but you seem to have your answer.
Really don't mind helping people but it takes a lot more time when you don't tell me what you have.

[69vetteside]

thanks tim,great info. It was and still is nite over here so in the morning when i get home from work i will look for that code and we will finally know what i really have in it.
cheers

[SS Tim]

Codes and charts can only tell us how it "was" built. No matter what the code on the axle says there could be anything inside after all these years. If you are getting that close spinning the wheel it is probably a 3.73. Just run the the tire size and RPM through a gear calculator to cross check it.

[69vetteside]

I guess it does'nt matter as i will be putting in the other diff with the 4.11. It was just out of curiosity.my 87 vette has 3.73 gears and the acceleration seemed roughly the same through the 1/2 gears.Thats all changed now that i put in the 427/550hp motor in the vette.

[Hawghauler]

I hope I'm not jacking this thread, but I guess I'm a little confused. If you have an open diff, jack up one tire, and do two rotations? Do you average the rotations? or do you add them together? And if added, wouldn't you then have 3.08:2
With the posi, jack both tires and do one rotation. One revolution to three on the yoke equals roughly 3.08:1, Three and three quarters equals roughly3.73:1, etc.

Thanks