Gear ratio

[Scott>72-GMC]

Besides the obvious opening the rear end is there another way to know what ratio I have ? It's a stock 72 3/4 ton gmc.
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[1972SUPERSB]

If you have a tach there is a calculations you can use using the tire height and RPM. Also you can jack up rear end of truck. Mark the driveshaft then turn one of the wheels 1 full revolution. Also notice how many times the driveshaft turns. This will get you pretty close. Example, tire does one complete turn driveshaft turns 3 times, probably 3.08 rear. If driveshaft turns between 3.5 and 4 then it's probably a 3.73
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[Sideways]

Yours being a 3/4 ton is likely 4:10 with a small block or 3:55 with a big block.
You can remove the diff cover and the number of teeth on the ring and pinion should be stamped on the ring gear,you can divide the big number by the small number to verify.

[Scott>72-GMC]

It's a 350 Thank you. I'm learning about gear differences I know there is probably a few but what are the main differences. Or one better than another?
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[Sideways]

4:10 gears are pretty low (higher rpm) for the highway but great for pulling or hauling a load.
Overdrive trannies seem to be a good cure to let the rpm drop when your cruising on the highway,just depends on what the truck is used for.

[LockDoc]

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There are a couple of ways to tell your current ratio. The one I usually use is the letters stamped on the housing. I have included a couple of pictures that show the location of the letters. You will have two or three letters, depending on the year of your truck. When you find the code letters post them and the year of your truck here and I can tell you what ratio you have.

Another good way is to take the cover off of the rear housing and look at the numbers stamped on the gears. The good thing about using this method is that you put fresh gear oil in the rear end to replace the 40 year old oil that is in there now. Below are the numbers for the different ratios. The last two numbers in each row are the ones that will be stamped into the gears.

Rear Axle Ratios

Ratio 2.56:1 - Tooth Combination 41,16
Ratio 2.73:1 - Tooth Combination 41,15
Ratio 3.07:1 - Tooth Combination 43,14
Ratio 3.08:1 - Tooth Combination 40,13
Ratio 3.08:1 - Tooth Combination 37,12
Ratio 3.31:1 - Tooth Combination 43,13
Ratio 3.36:1 - Tooth Combination 37,11
Ratio 3.42:1 - Tooth Combination 41,12
Ratio 3.55:1 - Tooth Combination 39,11
Ratio 3.73:1 - Tooth Combination 41,11
Ratio 4.10:1 - Tooth Combination 41,10
Ratio 4.11:1 - Tooth Combination 37,9

LockDoc

[Sideways]

Originally Posted by LockDoc

-
There are a couple of ways to tell your current ratio. The one I usually use is the letters stamped on the housing. I have included a couple of pictures that show the location of the letters. You will have two or three letters, depending on the year of your truck. When you find the code letters post them and the year of your truck here and I can tell you what ratio you have.

Another good way is to take the cover off of the rear housing and look at the numbers stamped on the gears. The good thing about using this method is that you put fresh gear oil in the rear end to replace the 40 year old oil that is in there now. Below are the numbers for the different ratios. The last two numbers in each row are the ones that will be stamped into the gears.

Rear Axle Ratios

Ratio 2.56:1 - Tooth Combination 41,16
Ratio 2.73:1 - Tooth Combination 41,15
Ratio 3.07:1 - Tooth Combination 43,14
Ratio 3.08:1 - Tooth Combination 40,13
Ratio 3.08:1 - Tooth Combination 37,12
Ratio 3.31:1 - Tooth Combination 43,13
Ratio 3.36:1 - Tooth Combination 37,11
Ratio 3.42:1 - Tooth Combination 41,12
Ratio 3.55:1 - Tooth Combination 39,11
Ratio 3.73:1 - Tooth Combination 41,11
Ratio 4.10:1 - Tooth Combination 41,10
Ratio 4.11:1 - Tooth Combination 37,9

LockDoc

Were all those ratio's available on '72 GMC 3/4 tons?

[LockDoc]

Originally Posted by Sideways

Were all those ratio's available on '72 GMC 3/4 tons?

No.

LockDoc

[LockDoc]

Originally Posted by 1972SUPERSB

If you have a tach there is a calculations you can use using the tire height and RPM. Also you can jack up rear end of truck. Mark the driveshaft then turn one of the wheels 1 full revolution. Also notice how many times the driveshaft turns. This will get you pretty close. Example, tire does one complete turn driveshaft turns 3 times, probably 3.08 rear. If driveshaft turns between 3.5 and 4 then it's probably a 3.73
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Originally Posted by 70CHEVYBB

jackit up,turn the wheel1 turn and count the turns of the drive shaft.

As far as finding the ratio by jacking the vehicle up there is actually more to it than that. You have to jack up one tire if you have an open differential, or both tires if you have a positraction or locking differential.

Rotate the tire one full revolution for positraction and lockers, and 2 full revolutions for open differentials. Count the number of full revolutions the driveshaft makes. This is your gear ratio. If the drive shaft turns just over 3 turns it should be a 3.08 if it turns 3 ¾ turns, it will be a 3.73 gear ratio.


LockDoc

[Scott>72-GMC]

I don't think it will be driven too much hoping to get a fifth wheel camper someday so sounds like I got what I need. Thanks ill give her a look when she shows up tomorrow
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[Sideways]

Originally Posted by Scott>72-GMC

I don't think it will be driven too much hoping to get a fifth wheel camper someday so sounds like I got what I need. Thanks ill give her a look when she shows up tomorrow
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Your welcome,good luck with it!

[SS Tim]

Well done Leon!

[LockDoc]

Originally Posted by SS Tim

Well done Leon!

Thanks Tim.

LockDoc

[70CHEVYBB]

jackit up,turn the wheel1 turn and count the turns of the drive shaft.

[Scott>72-GMC]

Just can't wait for it to show up tomorrow
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[Sideways]

Kinda like Christmas, just one more sleep!

[Mustard72]

I'm thinking a 72 C20 isn't going to have anything highway friendly. My Eaton is a 4.57, and I think the only other option was something like a 4.10 or 4.11 and maybe a 3.73 (although that might have been for the Dana rear???). Mine engine runs at 3200 rpm at 60 mph (at least it did with the 3OTT tranny that used to be in it), and that's with 29" tall tires.

A higher gear ratio(lower numerically, like 3.23s or something) would have the engine turning less rpm on the highway and give you better gas mileage and maybe less wear and tear on the engine, but you lose low end grunt for towing/hauling/impressing the ladies with effortless burnouts, etc.

[Scott>72-GMC]

Last time I drove it was bout 2 years ago I know it cruised along 65-70 maybe 75 I wasn't pushing it too hard I don't think. But around 65 the cab would shake my uncle and I are thinking the distributer is off a bit. Does it parked at high rpms too so I know it's not drive shaft. Anyway. I can't remember the rpms I was running when going that fast I know they weren't too low but not way up there either
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[eagleuh1]

Don't forget 4.56:1. Thats what my Dana 60 was in my 68 3/4 ton with 327. Put in a 3.54:1 out of a 71 3/4 ton and overcame the lower number ratio with a 5.3.

[Mustard72]

So the 3.54 must've been out of a Dana, too??? I'm trying to get a good understanding of this, too, as I may well be swapping for some more freeway friendly gears, too. What exactly all was available for the 3/4 tons? Did leafs or coils make a difference for which rear/gear was available?

Don't mean to hijack the thread. I think this sort of jives with what the OP was asking for, though.

[Scott>72-GMC]

Good questions
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[Scott>72-GMC]

Going by this with a turbo hydromatic and a 350 should be 4.10 or 4.57?
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[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
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[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