Stock Rear Axle Gearing

[King951]

Hey there,

My 1971 C20 that screams on the freeway. Was curious as to what the stock rear axle gear could be / how could I check? I didn't notice any tags on the rear axle other than the common stamped "60". It is a 1971 C20, 307 engine and 3 speed manual transmission. Is there an online resource of sort that shows the factory options as far as rear ends / gearing goes?

Thanks in advance.

EDIT: If it makes a difference my truck has both G50 & G70 order options for the heavy duty rear suspension springs / etc. https://imgur.com/K439yHm

[yuccales]

Guessing 4:10 at least, if not lower, maybe 4:56?

[Bigdav160]

307 and a 3 speed would have had a 4.56:1 as standard equipment.



Since you have "optional leaf springs" you might have a Dana 60 axle which opens up some options in gear ratios.

[hugger6933]

I'm betting on the Dana rear, he said all he saw was a 60 stamp

[Duck 37]

would those ratios be the same for a C10

[Mike C]

No.

[jocko]

Like BigDav said - 4.56. That's what it was born with unless there is an RPO on the SPID for something optional. Have you checked your SPID yet?

[Alex V.]

No, 3/4 ton with a small engine and manual trans. was prime territory for low gears. I like 4.10's, but 4.56's would make it wind pretty good unless it's sitting on 33's or 35's.

[franken]

The ratio will be stamped on the ring gear. It's almost certain the gear lube needs changed so why not have a look?

[Ironangel]

Originally Posted by franken

The ratio will be stamped on the ring gear. It's almost certain the gear lube needs changed so why not have a look?

Exactly, you can speculate until the cows come home and still have a 12-bolt that's a quart low with a posi that's on the verge of self destruction...Take a pic of the cover and post it, right before you go ahead and remove that cover to see what you have. All to often there will be fragments of busted thrust washers, pieces of pinion and or ring gear teeth, or nothing at all...Rear ends are probably the most neglected component in a trucks drive train. All three of my 12-bolts have "60" cast in the housing, but I dont think the C/20's got 12-bolt's. They either got Dana 60's or the HO72's. Once you've determined what rear you have go ahead and splurge for a couple of cover gaskets. I know many never use them but there just isn't a good reason not to and several reasons why you should. "Just Sayin" ~Ghostrider~

[outfield]

You can check the gear ratio by jacking up one tire, and rotating the tire 360 degrees, while counting the rotations of your pinion yolk. Just a tad over two pinion yolk rotations, you have 4:10's. two and one quarter (2 1/4) rotations of the yolk and you have 4:56 gears.

[kda2000]

What Outfield said. A little more info, normally when both wheels move (as the drive shaft moves the truck), for a 4.10 gear set the yoke will go around 4.10 times per full revolution of the tires. When one wheel on the ground (and one jacked up) then the motion on the yoke that is normally split between the two wheels all goes to the one that moves, and so the one wheel moves twice as far as it normally would. So to figure out what ratio you have, you spin the tire and count the revolutions of the yoke and multiple by two. To be a little more precise, mark the tire and spin it 10 times, count the turns on the yoke, and then multiple by two and divide by 10, and then you will get the integer number and the first digit behind the decimal point with precise accuracy. For example if you have 4.56 gears, and you spin the tire 10 times, the yoke should go around 22 1/2 time. So 22.5 *2 = 45; and 45 / 10 = 4.5 (the first two digits of 4.56 gears). This works for any gear ratio. Hope this helps.

[mike16]

a picture of your rear will get us headed in the right direction

[Chulisohombre]

Only way to know for sure is to look at the ring gear and see what the stamp says. You can also do the roster tire and count driveshaft revolutions. Just make sure to chock your front wheels before hand as you are putting the trans in neutral to do it. Most 3/4 tons I’ve seen are 4.10 or 4.56. But over the years the ratio could have been changed.
Posted via Mobile Device

[Mike C]

Unless someone has removed it, Dana axles have a ratio tag on one of the cover bolts. Assuming you have the Dana axle. Would have been nice if GM had done same...

[Chulisohombre]

Originally Posted by Mike C

Unless someone has removed it, Dana axles have a ratio tag on one of the cover bolts. Assuming you have the Dana axle. Would have been nice if GM had done same...

Gm axle ratios are sometimes on the spid. But that doesn’t mean it’s correct. Mine says 3.73 but I have swapped axles and now have 4.10.
Posted via Mobile Device

[Chulisohombre]

Have you looked at the glove box tag yet to see if it’s listed?
Posted via Mobile Device

[Bigdav160]

He posted a link to his SPID in post #1

[Mike C]

Originally Posted by Chulisohombre

Gm axle ratios are sometimes on the spid. But that doesn’t mean it’s correct. Mine says 3.73 but I have swapped axles and now have 4.10.
Posted via Mobile Device

The SPID just shows if someting was ordered different than the standard gear for whatever base combo the truck came with. I have not had any luck with GM's codes stamped on the axle tube being the same as what is inside the housing when I take it apart.

But the Dana tag is awesome as long as some mechanic doesn't not put it back. Or put if on something different... Like you, I have a truck that came with 3.73 gears on the SPID ('86 K30) and even had the Dana 3.73 tag on the front diff. But inside the case was a set of 4.10's.... in this case it would have been helpful if they left the tag off! (The Dana 70 in the rear with 4.10's did not have the tag on it anymore.

[Chulisohombre]

Originally Posted by Bigdav160

He posted a link to his SPID in post #1

Firgot that was posted.
Posted via Mobile Device

[Chulisohombre]

Originally Posted by Mike C

The SPID just shows if someting was ordered different than the standard gear for whatever base combo the truck came with. I have not had any luck with GM's codes stamped on the axle tube being the same as what is inside the housing when I take it apart.

But the Dana tag is awesome as long as some mechanic doesn't not put it back. Or put if on something different... Like you, I have a truck that came with 3.73 gears on the SPID ('86 K30) and even had the Dana 3.73 tag on the front diff. But inside the case was a set of 4.10's.... in this case it would have been helpful if they left the tag off! (The Dana 70 in the rear with 4.10's did not have the tag on it anymore.

After 40+ years anything can be changed. Not a lot still stock under mine except the transmission and front end housing.
Posted via Mobile Device

[Mike C]

Which is why I always tell people chances are it needs a fluid change anyhow. Just pull the cover, see what ya' got, change gasket and add new fluid!

[King951]

Well if it makes a difference for the past 3 days I have been working in San Dimas, CA which is about a 35 mile highway drive both ways. I have a tachometer and I try to keep the RPMs between 2500-2900 and I am getting passed by people all day long. Truck was not built for the highway, but it does great at everything else. I keep it at 2500-2900 as I am scared to go past 3k for an extended period of time. What do you think a safe cruising RPM range would be? Thanks.

[Mike C]

For a 35 mile trip, you will be fine anywhere under 3000 rpm. Bigger tires can drop the rpm more for you, but if you are serious about driving it any distance at freeway speeds, then either an OD trans or a set of 3.73 would be wise investment.

Once again, assuming you have the Dana rear. The Eaton rear the 4.10 gear is the lowest GM installed. I think it was Randy's Ring and Pinion had some 3.90 made years ago but they were big bucks.

If you can fab, a semi-float 14 bolt would be a great swap and fairly easy to find with 3.73 gears in a Suburban. (I put a 4.10 SF 14 in the back of my Jimmy.)

[King951]

Originally Posted by Mike C

For a 35 mile trip, you will be fine anywhere under 3000 rpm. Bigger tires can drop the rpm more for you, but if you are serious about driving it any distance at freeway speeds, then either an OD trans or a set of 3.73 would be wise investment.

Once again, assuming you have the Dana rear. The Eaton rear the 4.10 gear is the lowest GM installed. I think it was Randy's Ring and Pinion had some 3.90 made years ago but they were big bucks.

If you can fab, a semi-float 14 bolt would be a great swap and fairly easy to find with 3.73 gears in a Suburban. (I put a 4.10 SF 14 in the back of my Jimmy.)

How hard would that realistically hurt the towing / pulling power? It's not like I push this truck to extremes but I do occasionally have the bed loaded with demo (tiles / carpet / wood / etc.) and have to pull my small-medium sized double axle trailer. I wouldn't mind a lower gear and better highway speed, but with a 307 / 3-speed combo I also don't wanna run into a situation where I can't shift into 3rd gear. Check out this newest album, you can see a typical load and my trailer in the background. Thanks for all the feedback so far.

https://imgur.com/a/Ps5hZ