3.08 to 3.42

[Jerryred94silvy]

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?

 

[b454rat]

I have a Tahoe that has 3.42s in it. IMO it’s a perfect ratio. I can do 100mph before it cuts out at 3k rpm. 70 is just about 2k. I debated on going to 3.73s, my Yukon has that and would be an easy swap. But I plan on going with a 4l80, and they don’t have as steep OD as the 4l60 does. You could prolly find a rear with those gears already n just swap the whole thing, cheaper n easier.

 

[PlayingWithTBI]

I went from 3.42 to 4.11, set pinion depth (with a crush sleeve eliminator kit), backlash, and preload in my 10 bolt. I wish I had bit the bullet and did a true track too. I gained 3/10th of a second in 0-60 and 1/4 mile. I didn't see an appreciable reduction in mileage. I have 275/60 R15s too. at 60 MPH I'm turning ~2100 with a 700R4 trans. Here's what I used with prices from 2 years ago from my build spreadsheet.

Crush Sleeve eliminator YGA-55048 Yukon Gear Summit Racing $33.06
Rear End Overhaul Kit ZKGM8.5 Yukon Gear Amazon $57.00
4.11 ring & Pinion ZG GM8.5-411 USA Standard Gear Amazon $145.11

 

[DerekTheGreat]

What RPM are you turning at 70mph? The tire you're running is a bit shorter overall than the factory size. My wife has a '92 C1500, 350 five speed truck. Factory size, P235/75R15's. It was birthed with a 3.08 and we changed it to a 3.42 G80 ten bolt. However, with that rear end it turns about 2300RPM @ 70 whereas my '89 K1500 turns about 2100RPM. My truck will net just over 17mpg if you run 90%+ highway whereas hers only pulls just over 16. My truck is also a five speed, they've got the same transmission. Everything I've read in the past suggests 4x4 trucks will do about 1mpg worse than a comparable 4x2 truck yet not in my case. Perhaps it's the difference in cruising RPM that is causing the difference. Part of me wishes I just kept the 3.08 in there but the 3.42 has definitely been nicer for the city and that's mostly what she does when she drives it.

 

[Jerryred94silvy]

What RPM are you turning at 70mph? The tire you're running is a bit shorter overall than the factory size. My wife has a '92 C1500, 350 five speed truck. Factory size, P235/75R15's. It was birthed with a 3.08 and we changed it to a 3.42 G80 ten bolt. However, with that rear end it turns about 2300RPM @ 70 whereas my '89 K1500 turns about 2100RPM. My truck will net just over 17mpg if you run 90%+ highway whereas hers only pulls just over 16. My truck is also a five speed, they've got the same transmission. Everything I've read in the past suggests 4x4 trucks will do about 1mpg worse than a comparable 4x2 truck yet not in my case. Perhaps it's the difference in cruising RPM that is causing the difference. Part of me wishes I just kept the 3.08 in there but the 3.42 has definitely been nicer for the city and that's mostly what she does when she drives it.

The interwebz calculator says I should be turning over 1800 or so RPM at 70 mph (my truck doesn’t have a tach, which is annoying). And I do a lot more city driving than I thought I would so I’m sure the 3.42 gears will help with that like you said. I also might go back to my 16inch rims with a slightly taller tire in the future.

 

[arrg]

3.08 to 3.42 is a 11% increase in gear ratio. 3.08 to 3.73 is a 21% increase. Your 1800 RPM cruise would become 1998 RPM with 3.42 or 2178 with 3.73.

 

[Nickpisp]

Personally, I wouldn’t spend the money for that small of a change. I’d go 3.73’s at the minimum. I just went from 3.42’s to 4.10’s and a Truetrac.

 

[Stumpy]

I had an S10 that had 3.08/V6, and got the rear end out of another S10 that had 3.42s. Made a very noticeable difference on that truck. It wouldn't get out of it's own way before, and would do some impressive burnouts after. lol

That truck was a '96, and after the swap, it was enough of a difference to mess with the ABS. Lucked up and found a used Predator tuner, that allowed me to enter tire and gear info, which straightened it right out.

 

[PlayingWithTBI]

What RPM are you turning at 70mph?

I went out and checked while tuning some, it's around 2450RPM @ 70MPH which is right around where my power curve starts.

 

[Jerryred94silvy]

Personally, I wouldn’t spend the money for that small of a change. I’d go 3.73’s at the minimum. I just went from 3.42’s to 4.10’s and a Truetrac.

Well a tru trac isn’t a for sure for me yet. And the ring and pinion set by itself on summit is only $130 or so. And this is my do everything truck...because it’s my only truck lol. So I don’t want crazy off road/drag racing gears that make driving on the highway a nightmare.