how to tell gear ratio? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

crosby · 12-02-2006, 05:04 PM

I am trying to determine the gear ratio on a parts truck I have, a 1978 GMC K35, which has a dana 60 front and 14 bolt rear. I pulled the rear diff cover off, and can't even see the pinion teeth. I suppose I could jack up one side and spin the ring to count those teeth, but then what? I don't see any obvious markings anywhere on the housing, or anywhere on the truck? Thanks guys!

motopatzo · 12-02-2006, 05:39 PM

Quite a few years ago I wanted to find out an axle ratio quickly in a truck with no markings and it was at the seller's home so..........I jacked up the back of the truck (use jackstands to be safe) and marked the bottom of the drive shaft and the bottom of one rear wheel. Spin the drive-shaft one complete revolution while watching the rear wheel that is turning. It should move less than a full turn. If your wheel only moved 1/4 turn from your mark, your formula would be 1 divided by .25 = 4 or in other words 4:1 ratio. Try and be as accurate as you can in counting the wheel movement. This should put you in the ball park.

b454rat · 12-02-2006, 05:45 PM

Check the glovebox for the rpo codes, if the tag is still there. I've seen them undre the hood too. If neither are there, look for a tag on the front axle, it will be bolted with the cover bolt. It will say the number of teeth and the ratio. If that isn't there, pull the cover and look for the numbers there.

1969 GMC · 12-02-2006, 08:58 PM

Quote:

Originally Posted by crosby

I am trying to determine the gear ratio on a parts truck I have, a 1978 GMC K35, which has a dana 60 front and 14 bolt rear. I pulled the rear diff cover off, and can't even see the pinion teeth. I suppose I could jack up one side and spin the ring to count those teeth, but then what? I don't see any obvious markings anywhere on the housing, or anywhere on the truck? Thanks guys!

it should be stamped on the ring gear. try rotating it around til you can see it.

crosby · 12-02-2006, 11:46 PM

wow thanks guys, lots of good info there. appreciate it, I will let you all know how I make out.

magnethead · 12-03-2006, 01:32 PM

K35 dually should be 4.10's. I'm yet to find a dually with 3.73's from the factory.

12-02-2006, 05:04 PM	#1
crosby Registered User Join Date: Aug 2004 Location: New Hampshire Posts: 437	how to tell gear ratio? I am trying to determine the gear ratio on a parts truck I have, a 1978 GMC K35, which has a dana 60 front and 14 bolt rear. I pulled the rear diff cover off, and can't even see the pinion teeth. I suppose I could jack up one side and spin the ring to count those teeth, but then what? I don't see any obvious markings anywhere on the housing, or anywhere on the truck? Thanks guys! __________________ Crosby 1978 Chevrolet K10 stepside NH

12-02-2006, 05:39 PM	#2
motopatzo Registered User Join Date: Oct 2006 Location: maple ridge, b.c., canada Posts: 190	Re: how to tell gear ratio? Quite a few years ago I wanted to find out an axle ratio quickly in a truck with no markings and it was at the seller's home so..........I jacked up the back of the truck (use jackstands to be safe) and marked the bottom of the drive shaft and the bottom of one rear wheel. Spin the drive-shaft one complete revolution while watching the rear wheel that is turning. It should move less than a full turn. If your wheel only moved 1/4 turn from your mark, your formula would be 1 divided by .25 = 4 or in other words 4:1 ratio. Try and be as accurate as you can in counting the wheel movement. This should put you in the ball park.

12-02-2006, 05:45 PM	#3
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,462	Re: how to tell gear ratio? Check the glovebox for the rpo codes, if the tag is still there. I've seen them undre the hood too. If neither are there, look for a tag on the front axle, it will be bolted with the cover bolt. It will say the number of teeth and the ratio. If that isn't there, pull the cover and look for the numbers there. __________________ 2000 GMC CCSB 454/4L80 6” lift SAS 1999 Chevy CCSB 454/NV4500 6” lift 1999 GMC Yukon 350/4L60 4" SAS

12-02-2006, 11:46 PM	#5
crosby Registered User Join Date: Aug 2004 Location: New Hampshire Posts: 437	Re: how to tell gear ratio? wow thanks guys, lots of good info there. appreciate it, I will let you all know how I make out. __________________ Crosby 1978 Chevrolet K10 stepside NH

12-03-2006, 01:32 PM	#6
magnethead 7000 #'s of american IRON Join Date: Dec 2004 Location: north tx Posts: 883	Re: how to tell gear ratio? K35 dually should be 4.10's. I'm yet to find a dually with 3.73's from the factory.



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