Easiest way to know gear ratio?!!! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72dreamer · 03-19-2003, 02:06 AM

What is the easiest way to know the gear ratio of my truck. '72 C-20 402 bbc th400. I want to change them if they are not what will give me best performance.

Please help me do this all over the place with my newly rebuilt Edelbrock throuhgout 402

mikep · 03-19-2003, 02:13 AM

What sort of rear end do you have? If its a C20 you have either a dana 60 or an eaton. 90% of Danas are 4.10's but there should be a tag on the rear that says the ratio and gear count. A big block auto may have 3.54's. If its an eaton you'll have to have someone turn the wheel while you count driveshaft revolutions. Remember to chock one wheel if its a open rear ( non posi ) If its an open rear multiply the driveshaft revs by 2 to get your gear ratio. If its a posi or locker than just count revs. Its more precise if you do ten wheel revs. make sure to mark the driveshaft with chalk to make it easier to keep track of revolutions.

**Classic Heartbeat** · 03-19-2003, 11:32 AM

If it is the dana 60 that nobody has messed with, it should have a tag bolted to one of the bolts on the inspection cover that will tell you what the ratio is. Most trucks with your combination came out with 3.54 Dana's in them, but that is not always true. Especially with the custom ordered trucks. If it is the eaton rear end (best identified by the round inspection cover and the drop out front gear set), there will be a set of numbers stamped in the front housing on the top cooling fin on the right hand side of the gear housing. If you have the first three letters I can get you the ratio by looking in my GM parts catolog. Then last but not least, if you have a possi, or track lock rear end the ratio should be posted on the glove box option sheet. WES www.ClassicHeartbeat.com

*MJF* · 03-19-2003, 11:40 AM

If you want to figure out a ballpark number, just mark the pinion & turn the tire & count the # of turns. If you want to be exact pull the cover & get the #'s. Your truck could probably use the gear oil change anyway.

1972chevy4x4 · 03-19-2003, 02:04 PM

I vote on pulling cover and get numbers off of gears, and treating rear end to new oil.

Longhorn Man · 03-20-2003, 12:36 AM

I second the oil chainge. And if you can't find the numbers on the gears, counting teeth is pretty easy.

72dreamer · 03-21-2003, 12:51 AM

Thanks for all of the great help. This board is the best. You guys are all great. I will do it the "hard" way and pull the cover. I don't know what weight to put in... 80w-90????

Longhorn Man · 03-21-2003, 12:58 AM

80/90 is good.

03-19-2003, 02:06 AM	#1
72dreamer Registered User Join Date: Sep 2002 Location: St. George, Utah Posts: 18	Easiest way to know gear ratio?!!! What is the easiest way to know the gear ratio of my truck. '72 C-20 402 bbc th400. I want to change them if they are not what will give me best performance. Please help me do this all over the place with my newly rebuilt Edelbrock throuhgout 402 __________________ 72 c20 custom deluxe Rebuilt and hopped up 402 BBC Just the body to go...

03-19-2003, 02:13 AM	#2
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	What sort of rear end do you have? If its a C20 you have either a dana 60 or an eaton. 90% of Danas are 4.10's but there should be a tag on the rear that says the ratio and gear count. A big block auto may have 3.54's. If its an eaton you'll have to have someone turn the wheel while you count driveshaft revolutions. Remember to chock one wheel if its a open rear ( non posi ) If its an open rear multiply the driveshaft revs by 2 to get your gear ratio. If its a posi or locker than just count revs. Its more precise if you do ten wheel revs. make sure to mark the driveshaft with chalk to make it easier to keep track of revolutions. __________________ No truck :-(

03-19-2003, 11:32 AM	#3
Classic Heartbeat Project Junkie! Fishing Poor!! Join Date: Dec 1999 Location: Olympia,Wa. 98512 Posts: 10,770	If it is the dana 60 that nobody has messed with, it should have a tag bolted to one of the bolts on the inspection cover that will tell you what the ratio is. Most trucks with your combination came out with 3.54 Dana's in them, but that is not always true. Especially with the custom ordered trucks. If it is the eaton rear end (best identified by the round inspection cover and the drop out front gear set), there will be a set of numbers stamped in the front housing on the top cooling fin on the right hand side of the gear housing. If you have the first three letters I can get you the ratio by looking in my GM parts catolog. Then last but not least, if you have a possi, or track lock rear end the ratio should be posted on the glove box option sheet. WES www.ClassicHeartbeat.com __________________ On-line catolog Classic Heartbeat Pickup Parts WEHEPP@comcast.net Call us Toll Free (888) 338-2502 Like Us on Facebook No appointment necessary. 72 BB Chevy Custom Deluxe / Custom Camper 67 I-6 (Soon to be 5.7 LS1) Panel 68 BB Chevy Short Bed 72 6.0 LQ9 Short Bed Chevy 4X4 Olympia's fastest growing truck parts supplier. Olympia's home for Wayward Chevys

03-19-2003, 02:04 PM	#5
1972chevy4x4 Registered User Join Date: Feb 2003 Location: Wichita Kansas Posts: 223	I vote on pulling cover and get numbers off of gears, and treating rear end to new oil. __________________ 1972 chevy k10 4x4 longbed 4-inch lift, 3.73 front/rears, frame off nearly complete 1972 chevy k10 4x4 longbed parts truck, known as the "ugly green machine" 1990 GMC 3/4 shortbed ext cab Always in need of 67-72 parts in central ks.

03-21-2003, 12:51 AM	#7
72dreamer Registered User Join Date: Sep 2002 Location: St. George, Utah Posts: 18	Thanks for all of the great help. This board is the best. You guys are all great. I will do it the "hard" way and pull the cover. I don't know what weight to put in... 80w-90???? __________________ 72 c20 custom deluxe Rebuilt and hopped up 402 BBC Just the body to go...

03-19-2003, 11:40 AM	#4
MJF No longer truckless Join Date: Mar 2002 Location: Campbell, CA Posts: 299	If you want to figure out a ballpark number, just mark the pinion & turn the tire & count the # of turns. If you want to be exact pull the cover & get the #'s. Your truck could probably use the gear oil change anyway.

03-20-2003, 12:36 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	I second the oil chainge. And if you can't find the numbers on the gears, counting teeth is pretty easy.

03-21-2003, 12:58 AM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	80/90 is good.



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