Rear End Change Question

Last Call · Apr 9, 2022

wore out said: ↑

This truck was a 15 over and 3.36 on 11R24.5 from factory. I thought I would hate it however I friggin loved it. 9th for anything 2 lane up to 70 mph and the big hole for the big road. So at a million the 15 was giving trouble so I took the 18 out of my wrecked truck since it was almost new. (Didn’t last long probably due to being towed with driveline in truck). Ran it a few months and the front diff gave up. Not liking the 3.36 and 18 combo and already having 3.70 in the wrecked truck I swapped them. That suited me fine. One reason I went with what I had however was a cracked front housing and 402 housings of any sort are like hens teeth. A friend of mines truck burnt and he had low air same as mine with super 40’s with really recent 3.55’s. So a deal was struck and I rolled housings suspension and all under my frame.

Sounds like a lot of work and it was but it’s part of running older iron with hard to come by parts. Sometimes you do what you have to to keep moving till you nail down the permanent fix. With a cracked housing it could have went multiple ways. My frame is too nice to need a cut off and at 275 WB it’s all I can do to get on a 70ft scale so stretching it was out of the equation as well. What it did do is make me realize a few hard truths that even I before all that would have argued the point.

It’s top speed which doesn’t matter is actually faster now than the 3.36 and 15 over. The 3.70 and .73 overdrive was actually #### near dead nuts the same as 3.36 and .79. But there was the 1 situation in a hard cross wind you were gonna slow down or spin the sheet out of it. With the 3.55’s it’s better there.

MPG is as always 4.8-5.2
Top speed on Beaumont just east of witchita is 65 usually as always. Wind can affect that.
I will agree it does sound better at 75mph at 1500RPM.
For a 31 year old sheet box pulling a spread axle bovine bus it does ok.
Click to expand...

I'am glad to read your experience and your thoughts because that just made me feel better about the choice I made switching the rears 1st before I went the tranny route....
I've been thinking to myself the whole time I was doing this .. Why am I doing the hard part 1st .. but like you I bought a laided over truck from a friend and I knew what I was getting .. I didn't want to cut the frame on this truck either.. it's in to good of shape to chop up

Cornbread914 · Apr 9, 2022

wore out said: ↑

This truck was a 15 over and 3.36 on 11R24.5 from factory. I thought I would hate it however I friggin loved it. 9th for anything 2 lane up to 70 mph and the big hole for the big road. So at a million the 15 was giving trouble so I took the 18 out of my wrecked truck since it was almost new. (Didn’t last long probably due to being towed with driveline in truck). Ran it a few months and the front diff gave up. Not liking the 3.36 and 18 combo and already having 3.70 in the wrecked truck I swapped them. That suited me fine. One reason I went with what I had however was a cracked front housing and 402 housings of any sort are like hens teeth. A friend of mines truck burnt and he had low air same as mine with super 40’s with really recent 3.55’s. So a deal was struck and I rolled housings suspension and all under my frame.

Sounds like a lot of work and it was but it’s part of running older iron with hard to come by parts. Sometimes you do what you have to to keep moving till you nail down the permanent fix. With a cracked housing it could have went multiple ways. My frame is too nice to need a cut off and at 275 WB it’s all I can do to get on a 70ft scale so stretching it was out of the equation as well. What it did do is make me realize a few hard truths that even I before all that would have argued the point.

It’s top speed which doesn’t matter is actually faster now than the 3.36 and 15 over. The 3.70 and .73 overdrive was actually #### near dead nuts the same as 3.36 and .79. But there was the 1 situation in a hard cross wind you were gonna slow down or spin the sheet out of it. With the 3.55’s it’s better there.

MPG is as always 4.8-5.2
Top speed on Beaumont just east of witchita is 65 usually as always. Wind can affect that.
I will agree it does sound better at 75mph at 1500RPM.
For a 31 year old sheet box pulling a spread axle bovine bus it does ok.
Click to expand...

Curious, why would you need to cut your frame?

Cornbread914 · Apr 9, 2022

Catmando said: ↑

I'am glad to read your experience and your thoughts because that just made me feel better about the choice I made switching the rears 1st before I went the tranny route....
I've been thinking to myself the whole time I was doing this .. Why am I doing the hard part 1st .. but like you I bought a laided over truck from a friend and I knew what I was getting .. I didn't want to cut the frame on this truck either.. it's in to good of shape to chop up
Click to expand...

I asked the other guy this as well, so just curious, why would you have to cut the frame??

wore out · Apr 9, 2022

Cornbread914 said: ↑

Curious, why would you need to cut your frame?
Click to expand...

Some would say it’s easier to just put a cut off on it instead of going the way I did. I probably wouldn’t argue that point....done right I have nothing against a stretch. But no sense in cutting one in two if it’s not gonna gain anything.

AModelCat · Apr 9, 2022

Cut the frame off in front of the diffs on your truck and the donor. Weld the donor section onto your truck.

Usually the section of frame around the diffs is the worst, condition-wise, on older trucks. Very few people take the time to wash the frame around there thoroughly, so it tends to pit pretty badly.

kranky1 · Apr 11, 2022

Swine hauler said: ↑

No.

Your motor is making the power.....not your rear gears.
If you swap your rears you will only change your start ability (1 gear, getting started), and your top speed at a given RPM. All other driving will be in a half gear lower (less than that actually, going from 3.70 to only 3.55). In other words hitting a hill you may pull it in 8th instead of 9th. Etc.......... Your power stays constant, and it will only fall on its face if you fail to use your gearbox for it's intended purpose.

What wore out said was correct. I just explained it a little differently . I think.

Also, you may go slightly faster in reverse.
Click to expand...

Slightly? Backing up a set of trains with a 13spd and 3:55’s was a bit of a challenge. I swapped for a 15od and even empty I back them in deep reduction. Low range is still just a little quick

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Rear End Change Question

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Rear End Change Question

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