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Re gearing confusion

ETHAN5438
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ETHAN5438 Enthusiast

ETHAN5438

Posted
Posted

Just bought a 2005 1500 which has a 1996 2500 rear end with what I think is 4.56 ratio. The truck currently has 33s and to go 75 I’m at about 95mph on my cluster and  2800 rpm just eating through gas. What ratio would be best for mpg? I know the stock ratio is a 3.42 and I don’t really feel like charging the front ratio also causes it’s expensive 

CamGTP Senior Enthusiast

CamGTP

Posted
Posted

If the front ratio is 3.42's, then make the rear ratio 3.42 as well. Then your 4WD will work again.

 

3.42's or 3.73's are usually the best for mileage in a 4 speed auto truck.

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted (edited)

Yeah, 4.56s is like running 4.10s with a 35" tire.  

 

Mathematically, 4.56s through a 4L60 and 33 inch tires would turn 2,502 RPMs at 75 MPH.  2,802 rpms is doing 84 mph in 4th.

 

3.42s will deliver 1,875 rpms at 75 mph with those 33s.  Put stock 31s back on with 3.42s and she'll turn 1,967 rpms at the same speed.

 

Slowing down saves gas too!

Edited by swathdiver
CamGTP Senior Enthusiast

CamGTP

Posted
Posted

If you want the 4WD to function you must have matching front and rear gear ratio's. If the front already is 3.42's, that is what you must install. Otherwise you have to redo both the front and rear gears to something different.

 

3.42's isn't the perfect gear ratio for 33's, but it would work and surely will help with your fuel mileage.

 

My opinion would be that 3.73's would be better for 33's but that would require you change gears in both axles to that.

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ETHAN5438 Enthusiast

ETHAN5438

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that is what I was thinking about doing if it would be worth going to 3.73 then later on replacing the front differtial or just going with the 3.42 and be done with it. I don't feel like replacing the transmission again. As of right now the front driveshaft is disconnected. 

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted

Forty twos is certainly easier and less costly.  The 73s of course will cost more and may not deliver increased fuel economy but would give more ooompf off the line.  In your shoes, I'd put 42s out back and put the front driveshaft back in.

ETHAN5438 Enthusiast

ETHAN5438

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Based upon the information I shared would I have to get a different carrier if I were to go with 3.42 because when I was looking I saw carriers for 4.10 and down and if I really do have 4.56 ratio I would need a new locker which is a lot so I hope I don’t run into that

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  • 2 weeks later...
swathdiver Senior Enthusiast

swathdiver

Posted
Posted
On 6/21/2022 at 9:25 AM, ETHAN5438 said:

Based upon the information I shared would I have to get a different carrier if I were to go with 3.42 because when I was looking I saw carriers for 4.10 and down and if I really do have 4.56 ratio I would need a new locker which is a lot so I hope I don’t run into that

Nope.  

 

https://demandaam.com/axle-results?vm=GM&q=8.6"+10+Bolt+-+Rear+Axle

ETHAN5438 Enthusiast

ETHAN5438

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Posted

I found a good deal for locker and gears for $650 when with out a locker it was 450 so I just got the new lsd

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