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Changing rear end gears

O

ohmymy111

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Middle of nowhere North Dakota
I have a 1994 C3500 8.4l 2wd non-dually, that has 4.10 gears. I really don't need the truck for pulling anything but a dog trailer. I would like to improve my mileage, right now without pulling the trailer I can get 14mpg, and drop to about 9mpg pulling the trailer and was thinking about swapping out the gears going down to 3.42.

Looking for pros and cons and feasibility opinions.
 
8.4l?! They make any rear ratio you could want for your axle. 3.42s in a 6.5l can get you 19-20mpg or you can do an A Team Turbo and gain power and econ

Source Unknown
 
ohmymy111 said:
I have a 1994 C3500 8.4l 2wd non-dually, that has 4.10 gears. I really don't need the truck for pulling anything but a dog trailer. I would like to improve my mileage, right now without pulling the trailer I can get 14mpg, and drop to about 9mpg pulling the trailer and was thinking about swapping out the gears going down to 3.42.

Looking for pros and cons and feasibility opinions.
Click to expand...

tanman_2006 said:
8.4l?! They make any rear ratio you could want for your axle. 3.42s in a 6.5l can get you 19-20mpg or you can do an A Team Turbo and gain power and econ

Source Unknown
Click to expand...

Pretty sure he means 7.4L which would be the 454 big block. As for gear ratio choices, you can go 3.08, 3.42, or even 3.73, but the 454 is a fuel thirsty pig anyway you slice it.
 
DOH typoed the 8.4L, yes it is a 7.4L. I am not unhappy getting 14mph, but do miss my 6.5TD which got me 23mph. Just looking to eek out whatever increase I can get. I know with the 7.4L it will never be great.
 
How heavy is this dog box you pull? Are you sure you don't already have 3.73's? Depending on the weight I would suggest 3.73's, IMO 3.42's is too much lugging for a 454.
 
You're doing well to get 9 mpg towing with a 7.4. I doubt that a gear change would get you more than 1 mpg increase while towing. That dog trailer must not weigh much.
 
3:42 is as low as you can go unless someone is making a new gear for them now??
I did what you are wanting to do, but did not keep track of MPGs. I did it because I don't like the engine wraping out that high all the time and to get the noise levels down. It accomplished both and I love it. It will still pull anything I need to.
Tanners been in it also, its a very smooth and quite truck.
 
Burning oil said:
3:42 is as low as you can go unless someone is making a new gear for them now??
I did what you are wanting to do, but did not keep track of MPGs. I did it because I don't like the engine wraping out that high all the time and to get the noise levels down. It accomplished both and I love it. It will still pull anything I need to.
Tanners been in it also, its a very smooth and quite truck.
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If it's a 10.5" 14 bolt, GM built several for fleets with the 3.08 gears and manual trans.
 
I've not heard of them. Seems like the "big" gear houses would offer it if they were available. IMO 3:08 would be to low for towing.
Would be good for a 1/2 ton truck SF axle.
 
Burning oil said:
I've not heard of them. Seems like the "big" gear houses would offer it if they were available. IMO 3:08 would be to low for towing.
Would be good for a 1/2 ton truck SF axle.
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I know for 100% that I seen a few back when I was trying to decide which gears to go with in the BURB, but I just looked and AMM doesn't offer them now. They didn't do a lot of them, mainly just for large fleets after MPG.
 
It'd be kind of cool to see what one drives like with 3:08 Thow a gear vendor OD on there and you be making diesel not burning it :rolleyes5:
 
Burning oil said:
It'd be kind of cool to see what one drives like with 3:08 Thow a gear vendor OD on there and you be making diesel not burning it :rolleyes5:
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I tried a GEAR VENDORS when I had my 6.5, got right at 20-21 out of it VS the 17-18 I got with my 4.10's. I now get 21 or so with 3.42's and the DURAMAX.
 
I'm guessing with the advent of overdrive transmission the low gear ratios have been discontinued. I also found some people with used 3.21 gear sets for them from the 80's.
 
The 3.21 ratio is what I saw. I remember it being a weird ratio. The guy that had it was actually a member here a yr or 2 ago but his 6.5L blow and he did a 12 V.
 
Do the ring & pinion gears from 84ish work in the 90-2000 14 bolts? Im to lazy to look up part #s this morn.
(see i was to lazy to even spell out MORNING)
 
Yep, so long as it is the GM 10.5" 14 bolt. AAM also makes another 10.5" 14 bolt now for DODGE, but those gears do not interchange with the GM's.
 
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