3.08 to 3.42

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straycat

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My truck ( '88 rcsb ) came with a 305 , 400 trans. , and a 3.08 gear. After a 350/ 700R4 swap, it was still a dog off the line. Got a 3.73 posi from the boneyard- what a difference! Money well spent!!
 

1989GMCSIERRA

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So after awhile of driving my truck and noticing it doesn’t quite have the “oomph” off the line I was expecting (it’s a 355 with heads cam and intake) I did some investigating and found that have 3.08 rear gears, which I thought was very odd considering that the truck originally had a 4.3 in it and must have been apalling off the line. The truck still does just fine, I would just like some more punch. So I’m considering switching to 3.42’s to get a little more down low and still not be screaming on the highway at 60 miles an hour lol. I’m running 275/60r15 tires on a 10 bolt 8.5 inch rear end.

So with all that being said, here comes the tsunami of questions...

Has anyone done this or a similar swap? If so, what results did you see? Will I need to set pinion depth with the new gears or not? If I do need to set it I suppose getting the master bearing kit is the way to go? And finally, is it worth it to throw an LSD in while I’m in there? I’ve heard tru tracs are the way to go. What do y’all think?

I wouldn’t waste the money on such small gear swap. You’re better off hitting the junkyard and finding a 3:73 GovLoc rear axle of your got a open rear.

I would go to say 3:73 if you aren’t planning on going bigger or lifting your truck. You can do 4:10s if you dint care about mpg and it’s a fun truck. You can always go to a 31/32 inch tire if you don’t care about the sport truck look. The 4:10s will work with those.
 

90halfton

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Swapped out my 3.08s to 3.42s when I rebuilt the rear of my 90 c1500. 350, auto, 235 tires. Made it far more drivable, pulled a trailer better (5k lbsish), and keeps a happy rpms at 75 unloaded in od. Gears are the best mod for these trucks. No supplement for mechanical advantage. If you want a tire burner id go at least 3.73 with reasonably sized tires. But 3.42 is a good size for a wake up in a truck that needs to go freeway speeds and still do work.
 

Jerryred94silvy

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Swapped out my 3.08s to 3.42s when I rebuilt the rear of my 90 c1500. 350, auto, 235 tires. Made it far more drivable, pulled a trailer better (5k lbsish), and keeps a happy rpms at 75 unloaded in od. Gears are the best mod for these trucks. No supplement for mechanical advantage. If you want a tire burner id go at least 3.73 with reasonably sized tires. But 3.42 is a good size for a wake up in a truck that needs to go freeway speeds and still do work.

This is my main focus really, living in San Antonio if I’m going anywhere it’s on the freeway. But it’s sounding like 3.73s will do just fine for that? I’m definetly not opposed to making a little smoke once in awhile (it oughta shut my Ford friends up pretty quick lol). I’m getting 12.5 mpg as it sits and would love if it stayed there and not get worse. But it’s a truck, and a fun truck at that. I always say “if I wanted fuel economy, I’d buy a corolla” so it’s not the end of the world if fuel economy suffers a little in the name of horsepower and fun.:headbang:
 

90halfton

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This is my main focus really, living in San Antonio if I’m going anywhere it’s on the freeway. But it’s sounding like 3.73s will do just fine for that? I’m definetly not opposed to making a little smoke once in awhile (it oughta shut my Ford friends up pretty quick lol). I’m getting 12.5 mpg as it sits and would love if it stayed there and not get worse. But it’s a truck, and a fun truck at that. I always say “if I wanted fuel economy, I’d buy a corolla” so it’s not the end of the world if fuel economy suffers a little in the name of horsepower and fun.:headbang:
Ya, 275 tires i don't think I'd go smaller than 3.73. Your mileage will more likely increase than decrease.
 

sewlow

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10 years after I bought my '98, I swapped the 3.42's/Gov-lock to a set of 3.73's with an Eaton LSD.
275/60-15's.
Should of done that the day after I bought the truck.
Sure woke it up, but it did get a bit thirstier.
That mpg hit could of (well...more than likely. Probably!) been due the increased yippee-yahoo factor. It was like a different truck. Way too much fun to hammer down on the loud pedal.
If I'd lightened up on the right foot around town, it would have probably made better mileage due to not having to be pushed as hard to keep up with traffic.
But then, I've never really had a big concern over mpg's. Smiles per mile has always been the priority.
 

Jerryred94silvy

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Well shoot. It sounds like I’ll be getting 3.73s and a tru trac then! I can certainly tell the truck lugs a bit up hills in overdrive on the highway, and around the city in stop and go those gears will definetly help. And also the yippee-yahoo factor is a major consideration lol.
 

stutaeng

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Well shoot. It sounds like I’ll be getting 3.73s and a tru trac then! I can certainly tell the truck lugs a bit up hills in overdrive on the highway, and around the city in stop and go those gears will definetly help. And also the yippee-yahoo factor is a major consideration lol.

What year is your truck? I seem to think that 88-94 TBIs had numerically lower gears vs. the later Vortecs? I don't know if it was because the TBIs had good low-end torque? My nephew's '90 C1500 F44 ECSB 350 had something like 2.73s. He did a LQ4/TH400 dragster swap and got regeared to 4.10. My brother's C1500 RCSB has 3.08 also. I don't know what my Dad's 89 1500 RCSB has, but probably something along those lines.

Didn't the later 1500 Vortecs have 3.42/3.73s mostly? Or maybe it was as the ECSB became more popular over the RCSB? Heavier truck requires numerically higher gears?
 

b454rat

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I had a couple TBI trucks, one I think had 3.08s, but with a 406 had NO problem pulling those gears, and then had a 92 Blazer that had 3.42s, that had 32s on it and was a bit of a dog, but it had 200k on it, so was easy on it. I had a 98 Tahoe that had 3.42s and 4wd, that had some pull to it, but nothing like the 98 that I have now that is 2wd with the same gears, that pulls great, but run out of MPH cause it shuts off at 100 lol.
 
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