What gear is it? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Hair · 03-19-2020, 07:07 PM

2wd - 71 GMC with a 250 inline and a powerglide. What gear is the rear end?

68Gold/white · 03-19-2020, 07:26 PM

Quote:

Originally Posted by Hair

2wd - 71 GMC with a 250 inline and a powerglide. What gear is the rear end?

Most likely a 3.73...

**truckster** · 03-19-2020, 07:39 PM

Is there a SPID on the inside of the glovebox door? It would at least tell you what the original gear ratio was.

**jocko** · 03-20-2020, 07:23 AM

If nothing is called out on the SPID, would have been 3.73 (standard). If anything else, it would have been on the SPID (options were 3.08 or 4.11) - and to truckster's point, that would only narrow down what it was born with, no guarantee that's what's in there today.

sick472 · 03-20-2020, 09:50 AM

Jack the rear up off the ground (with stands, of course), get a helper to assist in rotating the rear wheels together at the same time one full revolution (tranny in neutral). All the while counting the number of times the drive shaft turns. The number of drive shaft rotations equals the gear ratio.

Examples, if the drive shaft turns just barely over 3 turns when the rear wheels turn once...it's a 3.07.

Almost 3-3/4 turns...it's a 3.73.

If it is a posi-track you don't really need the helper as the tires should turn together in the same direction. If it is a single track it becomes important the both tires are turned the same direction one full turn. I mark tires and drive shaft with tape to help keep track of the turns. With the exception of the 4.10 and 4.11 gears, the rest of the ratios are far enough apart to acquire good confidence in your turn counts.

**truckster** · 03-20-2020, 10:47 AM

Or... since the gear oil is probably 50 years old, it might be due for a change. When you pull the cover, look at the numbers stamped into the ring gear. Divide the larger number by the smaller number and that will give you the gear ratio.

Example - if the numbers are 41 and 11, divide 41/11=3.73

**LockDoc** · 03-20-2020, 01:17 PM

-
If the axle tube or center section has the code stamped on it you can look it up in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Posts #8 & #19 show where the codes should be on the axle tube or center section, if it has any.

LockDoc

71gmcC15 · 03-20-2020, 01:25 PM

Mabe the 4 speed 307s where different but mine is listed on the spid (yes the 4 speed is not listed but is original had a hole thread about it.)

03-19-2020, 07:39 PM	#3
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,976	Re: What gear is it? Is there a SPID on the inside of the glovebox door? It would at least tell you what the original gear ratio was. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

03-20-2020, 07:23 AM	#4
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,975	Re: What gear is it? If nothing is called out on the SPID, would have been 3.73 (standard). If anything else, it would have been on the SPID (options were 3.08 or 4.11) - and to truckster's point, that would only narrow down what it was born with, no guarantee that's what's in there today.

03-20-2020, 09:50 AM	#5
sick472 Registered User Join Date: Jul 2015 Location: Sedalia Mo. Posts: 1,131	Re: What gear is it? Jack the rear up off the ground (with stands, of course), get a helper to assist in rotating the rear wheels together at the same time one full revolution (tranny in neutral). All the while counting the number of times the drive shaft turns. The number of drive shaft rotations equals the gear ratio. Examples, if the drive shaft turns just barely over 3 turns when the rear wheels turn once...it's a 3.07. Almost 3-3/4 turns...it's a 3.73. If it is a posi-track you don't really need the helper as the tires should turn together in the same direction. If it is a single track it becomes important the both tires are turned the same direction one full turn. I mark tires and drive shaft with tape to help keep track of the turns. With the exception of the 4.10 and 4.11 gears, the rest of the ratios are far enough apart to acquire good confidence in your turn counts. __________________ He who is without oil shall throw the first rod. Compressions 8.7:1 1972 C10 1976 C10 (parts truck) 1985 K20

03-20-2020, 10:47 AM	#6
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,976	Re: What gear is it? Or... since the gear oil is probably 50 years old, it might be due for a change. When you pull the cover, look at the numbers stamped into the ring gear. Divide the larger number by the smaller number and that will give you the gear ratio. Example - if the numbers are 41 and 11, divide 41/11=3.73 __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

03-20-2020, 01:17 PM	#7
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,774	Re: What gear is it? - If the axle tube or center section has the code stamped on it you can look it up in this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=578793 Posts #8 & #19 show where the codes should be on the axle tube or center section, if it has any. LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

03-19-2020, 07:07 PM	#1
Hair Registered User Join Date: Mar 2008 Location: Tullahoma, TN Posts: 406	What gear is it? 2wd - 71 GMC with a 250 inline and a powerglide. What gear is the rear end?

03-20-2020, 01:25 PM	#8
71gmcC15 Registered User Join Date: Feb 2018 Location: Doland SD Posts: 836	Re: What gear is it? Mabe the 4 speed 307s where different but mine is listed on the spid (yes the 4 speed is not listed but is original had a hole thread about it.) Attached Images __________________ ( second times the charm) 71 gmc c15 307 sbc 373 rear gears positrac sm 465 trans. ( not listed on spid but is original ) 94 chevy k1500 350 tbi 373 rear gears 4l60e one wheel peel. Gutless 07 gmc sierra classic rcsb 4.3 lt 5 speed w/t 04 chevy Silverado 1500 ext cab 5.3 4l60e. 81 chevy Scottsdale k10 muncie sm465 Np208 10 bolt 8.2 ( Old Green)

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