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

chevytrucks79 · 11-08-2008, 02:36 AM

Where are the directions to do this or how do I figure out my rear end ratio?

I have a '69 GMC Custom Camper. I am wondering because the other day I pulled a trailer (about 3 or 4K lbs) and the truck pulled it well but it basically topped out at 55 mph. I still had room to acellerate a little but the truck sounded better at that speed. I also got crappy gas mileage, about 8 mpg. My buddy, by guessing the way it drove, guessed it probally has 4.10. It is a heavy half ton from what I know. It has 6 lug wheels, 12 bolt rearend, 350 engine with edelbrock carb and intake and 350 transmission, rear leaf springs. The truck handled well with the trailer, I was actually surprised. I do have a little bit of play in the steering which was amplified with the load. I am going to add a front sway bar this weekend as well as overload springs for added strength and because the rear end sits slightly lower than the front. I figure this will raise it up a bit. I also want to sell my carb and get a quadrajet because I think it will improve my gas milage overall. I put new springs on it a couple of years ago and rebuilt my steering system.

JimKshortstep4x4 · 11-08-2008, 04:17 AM

8 miles per gallon pulling a trailer is not all that bad but hopefully you can improve the mpg overall.

There are other ways to check the ratio but what works pretty well is to mark one of the drums and one of the backing plates with chalk or similiar. Mark the rear yoke and housing with a line, also.

With the truck on level ground push the truck forward so that the brake drum makes one rotation while counting how many times the yoke rotates. You will need someone to assist you.

The ratio of the yoke turns to the drum turn is your rear end ratio. Full turns of the yoke are obvious but the partial turn can be estimated and compared to the known ratios for 12 bolt differentials.

Another method is to pull the cover and check the ring gear as they are normally stamped with the ratio.

Jim

Longhorn Man · 11-08-2008, 11:42 AM

no such thing as a heavy half or anything to that effect. That is just a 1/2 ton.
Your truck should have a 3.07 rear gear. Pull the cover off, and count the teeth on the drive gear, and then the driven gear, then divide the two.
Put fresh sealant or gasket on the cover, assemble, and top off with fresh oil.

11-08-2008, 02:36 AM	#1
chevytrucks79 Registered User Join Date: Oct 2004 Location: Richmond, VA Posts: 68	rear axle ratio Where are the directions to do this or how do I figure out my rear end ratio? I have a '69 GMC Custom Camper. I am wondering because the other day I pulled a trailer (about 3 or 4K lbs) and the truck pulled it well but it basically topped out at 55 mph. I still had room to acellerate a little but the truck sounded better at that speed. I also got crappy gas mileage, about 8 mpg. My buddy, by guessing the way it drove, guessed it probally has 4.10. It is a heavy half ton from what I know. It has 6 lug wheels, 12 bolt rearend, 350 engine with edelbrock carb and intake and 350 transmission, rear leaf springs. The truck handled well with the trailer, I was actually surprised. I do have a little bit of play in the steering which was amplified with the load. I am going to add a front sway bar this weekend as well as overload springs for added strength and because the rear end sits slightly lower than the front. I figure this will raise it up a bit. I also want to sell my carb and get a quadrajet because I think it will improve my gas milage overall. I put new springs on it a couple of years ago and rebuilt my steering system.

11-08-2008, 04:17 AM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: rear axle ratio 8 miles per gallon pulling a trailer is not all that bad but hopefully you can improve the mpg overall. There are other ways to check the ratio but what works pretty well is to mark one of the drums and one of the backing plates with chalk or similiar. Mark the rear yoke and housing with a line, also. With the truck on level ground push the truck forward so that the brake drum makes one rotation while counting how many times the yoke rotates. You will need someone to assist you. The ratio of the yoke turns to the drum turn is your rear end ratio. Full turns of the yoke are obvious but the partial turn can be estimated and compared to the known ratios for 12 bolt differentials. Another method is to pull the cover and check the ring gear as they are normally stamped with the ratio. Jim __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

11-08-2008, 11:42 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: rear axle ratio no such thing as a heavy half or anything to that effect. That is just a 1/2 ton. Your truck should have a 3.07 rear gear. Pull the cover off, and count the teeth on the drive gear, and then the driven gear, then divide the two. Put fresh sealant or gasket on the cover, assemble, and top off with fresh oil.



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