Gear ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Scott>72-GMC · 03-08-2013, 05:03 PM

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 · 03-08-2013, 05:13 PM

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 · 03-08-2013, 07:50 PM

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 · 03-08-2013, 07:57 PM

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 · 03-08-2013, 08:06 PM

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** · 03-08-2013, 08:19 PM

-
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

Scott>72-GMC · 03-08-2013, 08:22 PM

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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SS Tim · 03-08-2013, 08:25 PM

Well done Leon!

**70CHEVYBB** · 03-08-2013, 08:31 PM

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

Sideways · 03-08-2013, 08:32 PM

Quote:

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** · 03-08-2013, 08:33 PM

Quote:

Originally Posted by SS Tim

Well done Leon!

Thanks Tim.

LockDoc

**LockDoc** · 03-08-2013, 08:36 PM

Quote:

Originally Posted by Sideways

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

No.

LockDoc

**LockDoc** · 03-08-2013, 09:19 PM

Quote:

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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Quote:

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

Sideways · 03-08-2013, 09:29 PM

Quote:

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
Posted via Mobile Device

Your welcome,good luck with it!

Scott>72-GMC · 03-08-2013, 09:46 PM

Just can't wait for it to show up tomorrow
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Sideways · 03-08-2013, 09:53 PM

Kinda like Christmas, just one more sleep!

Mustard72 · 03-09-2013, 02:06 AM

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 · 03-09-2013, 02:19 AM

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 · 03-09-2013, 03:13 AM

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 · 03-09-2013, 12:35 PM

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 · 03-09-2013, 04:31 PM

Good questions
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**TBONE1964** · 03-09-2013, 04:36 PM

Quote:

Originally Posted by LockDoc

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

Thank you Leon, I was going to chime in on this as there is a lot of confusion out there about how many tire revolutions depending on posi/lockers and open diff's.

**LockDoc** · 03-09-2013, 05:09 PM

Quote:

Originally Posted by 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.

Yes, I think the 3.54 was a Dana gear only, I have one of those. I really can't help on your other two questions though.

Quote:

Originally Posted by TBONE1964

Thank you Leon, I was going to chime in on this as there is a lot of confusion out there about how many tire revolutions depending on posi/lockers and open diff's.

No problem TBONE. If a person is going to post about that method they need to post the whole story...

LockDoc

SS Tim · 03-09-2013, 06:12 PM

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.

Scott>72-GMC · 03-09-2013, 06:19 PM

Going by this with a turbo hydromatic and a 350 should be 4.10 or 4.57?
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03-08-2013, 05:03 PM	#1
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Gear ratio 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. Posted via Mobile Device __________________ 72 GMC super custom.

03-08-2013, 05:13 PM	#2
1972SUPERSB Registered User Join Date: Sep 2012 Location: San Antonio, TX Posts: 1,026	Re: Gear ratio 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 Posted via Mobile Device __________________ Jeff 1972 Cheyenne Super SWB 350, Auto, Bucket Seats w/Console, Tach, Factory A/C, P/S ,P/B, Tilt 1998 Chevy 1500 Ext Cab 5.7, Pewter w/blue interior

03-08-2013, 07:50 PM	#3
Sideways Registered User Join Date: Jul 2007 Location: Left Coast,Canada Posts: 294	Re: Gear ratio 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.

03-08-2013, 07:57 PM	#4
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio 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? Posted via Mobile Device __________________ 72 GMC super custom.

03-08-2013, 08:06 PM	#5
Sideways Registered User Join Date: Jul 2007 Location: Left Coast,Canada Posts: 294	Re: Gear ratio 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.

03-08-2013, 08:19 PM	#6
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,772	Re: Gear ratio - 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 Attached Images __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

03-08-2013, 08:22 PM	#7
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio 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 Posted via Mobile Device __________________ 72 GMC super custom.

03-08-2013, 08:25 PM	#8
SS Tim Registered User Join Date: Aug 2010 Location: Edwards, CA Posts: 7,503	Re: Gear ratio Well done Leon! __________________ Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID Blazer SPID Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders Do you have a 1972 Plaid Pickup? Plaid Blazer ** Plaid Suburban

03-08-2013, 08:31 PM	#9
70CHEVYBB Senior Member Join Date: Feb 2012 Location: chillicothe, illinois Posts: 690	Re: Gear ratio jackit up,turn the wheel1 turn and count the turns of the drive shaft.

03-08-2013, 09:46 PM	#15
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio Just can't wait for it to show up tomorrow Posted via Mobile Device __________________ 72 GMC super custom.

03-08-2013, 09:53 PM	#16
Sideways Registered User Join Date: Jul 2007 Location: Left Coast,Canada Posts: 294	Re: Gear ratio Kinda like Christmas, just one more sleep!

03-09-2013, 02:06 AM	#17
Mustard72 Registered User Join Date: Jul 2012 Location: Ladysmith, WI Posts: 641	Re: Gear ratio 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. __________________ Nick Old Mustard, 1972 Chevy C20, 350, TH350, 4.10 Dana 60 rear, owned since I was 16 in 1986 http://67-72chevytrucks.com/vboard/s...d.php?t=545356 Blue Bell, 1987 K20, 6.2L Diesel, TH400/NP208, 4.10 gears Ustahav 1976 K10, 350/350/203, Gov Loc, Yooper beater trail truck.

03-09-2013, 02:19 AM	#18
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio 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 Posted via Mobile Device __________________ 72 GMC super custom.

03-09-2013, 03:13 AM	#19
eagleuh1 Registered User Join Date: Jul 2009 Location: Russellville, Arkansas Posts: 1,699	Re: Gear ratio 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.

03-09-2013, 12:35 PM	#20
Mustard72 Registered User Join Date: Jul 2012 Location: Ladysmith, WI Posts: 641	Re: Gear ratio 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. __________________ Nick Old Mustard, 1972 Chevy C20, 350, TH350, 4.10 Dana 60 rear, owned since I was 16 in 1986 http://67-72chevytrucks.com/vboard/s...d.php?t=545356 Blue Bell, 1987 K20, 6.2L Diesel, TH400/NP208, 4.10 gears Ustahav 1976 K10, 350/350/203, Gov Loc, Yooper beater trail truck.



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03-09-2013, 04:31 PM	#21
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio Good questions Posted via Mobile Device __________________ 72 GMC super custom.

03-09-2013, 06:12 PM	#24
SS Tim Registered User Join Date: Aug 2010 Location: Edwards, CA Posts: 7,503	Re: Gear ratio 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. Attached Images __________________ Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID Blazer SPID Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders Do you have a 1972 Plaid Pickup? Plaid Blazer ** Plaid Suburban

03-09-2013, 06:19 PM	#25
Scott>72-GMC Registered User Join Date: Jan 2013 Location: New providence ia Posts: 1,018	Re: Gear ratio Going by this with a turbo hydromatic and a 350 should be 4.10 or 4.57? Posted via Mobile Device __________________ 72 GMC super custom.

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