Diff gears
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Senior User
Joined: Jul 2015
Posts: 102
Likes: 1
From: Victoria, Australia.
Diff gears
Hi there I've got a 351 with a c6 and a nine inch with 3.4 gears just wandering is that a low range ratio or is that really tall? Truck doesn't quite get up and go as much as I would like
FTE Legend
Joined: Nov 2015
Posts: 30,856
Likes: 4,106
From: Angier, NC
Are you sure on the 3.4 ratio for the 9"?
I ask because a quick look and I don't see that ratio.
Lets call it a 3.50 ratio. Now is that a tall (high way gear) or short (off high way gear) is hard to say with out other information of what the gear is in, what is the weight of it, what will it be used for, tire size, motor spec (cam spec) just to name a few.
For a not too heavy truck, non over drive trans and say a 27" tall tire that is not too bad of a ratio.
Now add either a over drive trans or 35" tall tire and the ratio would be too tall and a 4.11 or even a 4.56 gear would be better.
Add just the 35" tall tires and want to use the truck off road most of the time you may want to go even shorter on the gear.
So as you can see there is no good answer.
Dave ----
I ask because a quick look and I don't see that ratio.
Lets call it a 3.50 ratio. Now is that a tall (high way gear) or short (off high way gear) is hard to say with out other information of what the gear is in, what is the weight of it, what will it be used for, tire size, motor spec (cam spec) just to name a few.
For a not too heavy truck, non over drive trans and say a 27" tall tire that is not too bad of a ratio.
Now add either a over drive trans or 35" tall tire and the ratio would be too tall and a 4.11 or even a 4.56 gear would be better.
Add just the 35" tall tires and want to use the truck off road most of the time you may want to go even shorter on the gear.
So as you can see there is no good answer.
Dave ----
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Senior User
Joined: Jul 2015
Posts: 102
Likes: 1
From: Victoria, Australia.
Hi there the truck is a longbed 2WD bone stock 351C with C6 auto, 31" BFG AT tyres, and a LSD, and it is un loaded empty tub, it just feels like when u floor it it revs descently but it just doesn't move very fast.
The way I found out the ratio, I turned the tyre 10 times and got someone to watch how many times the driveshaft turned, so approximately the wheels turned 10 times and the driveshaft turned 34-35 times, so I'm guessing the ratio is 3:4 3:5????
The way I found out the ratio, I turned the tyre 10 times and got someone to watch how many times the driveshaft turned, so approximately the wheels turned 10 times and the driveshaft turned 34-35 times, so I'm guessing the ratio is 3:4 3:5????
Lead Driver
Joined: Apr 2007
Posts: 5,958
Likes: 105
From: Houston
Johno.h have you looked at the tag on the front of your diff? most of the time the tag is still there generally on the passengerside at what would be 10:30 position. Might have to clean the tag but it tells you what gear ratio it is.
My '82 the tag says 2 75 on mine.
My '82 the tag says 2 75 on mine.
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
Or, decode the Certification Label that is on the driver's door post - assuming they did it that way for Australia. And, that the decoder works for there. Anyway, here, my decoder: Certification Label - Gary's Garagemahal.
As for 3.50:1 being high or low, that was the standard gear for 351 CI engines in the States. All 5 of these trucks I've owned have had it, and I find it is too low of a gear for continued highway usage when combined with a non-OD tranny. But stick something like a ZF5 in front of it and it'll loaf at highway speeds while still having enough oomph for towing.
As for 3.50:1 being high or low, that was the standard gear for 351 CI engines in the States. All 5 of these trucks I've owned have had it, and I find it is too low of a gear for continued highway usage when combined with a non-OD tranny. But stick something like a ZF5 in front of it and it'll loaf at highway speeds while still having enough oomph for towing.
Lead Driver
Joined: Apr 2007
Posts: 5,958
Likes: 105
From: Houston
That's right, but what if the rear axle was changed at one time or the carrier was changed at one time due to damage. In a perfect world the real shops would affix the proper tax from the rearend to what ever vehicle it is installed in.
We have one at work its a '82 F150 I did the Coyote engine in, this thing the door tag says its the 2.75:1 axle ratio but the tag on the rear end says its a 3.50:1. I know his rearend was changed out before and he said that when it was changed out it went with a "faster" rearend as he called it to pull all the extra weight he had loaded down in the bed of his truck.
We have one at work its a '82 F150 I did the Coyote engine in, this thing the door tag says its the 2.75:1 axle ratio but the tag on the rear end says its a 3.50:1. I know his rearend was changed out before and he said that when it was changed out it went with a "faster" rearend as he called it to pull all the extra weight he had loaded down in the bed of his truck.
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