Need opinions: 3.42 vs 3.73 GR

DaveO9 · May 24, 2024

I know I want to change out the 3.08s in my '13, 5.3, 4wd Tahoe, just not sure if I want 3.42 or 3.73. Some factors I'm considering:

- I don't tow anything heavy yet, but my wife and I are considering a small travel trailer. Upper weight limit would be about 5K.
- Would like to stay with the 5.3, without major mods. (not opposed to a cam swap and related. Still have AFM valvetrain, disabled in tune.)
- Hills are a factor in the PNW. Just about any place you go with a trailer will involve significant grades and probably mountain passes.
- Don't want a screamer going down the freeway unloaded at 75 mph. Also don't want to cause big impacts to unloaded highway mpg. (But I'm pretty unclear as to how much effect several hundred rpm has. I've tried some experiments by shifting to 5th and then matching what my rpm would be with different gears, but I think I'm just seeing effects from small grade changes. The experiments did show me, however, that it's not a big difference in noise between 1600 rpm and 2000 rpm.
- rather than swapping R&P, I plan to just buy junkyard compete assemblies, front and rear. Most units I'm looking at have fewer miles than my current ones. But....
- I'd really like to do the upgrade to the 9.5"/14-bolt rear. This adds several hundred to the cost for the 3.42 ratio, but even more for the 3.73 since they never used that ratio with that assembly. I'd have to do a R&P swap if I want both 14 bolt and 3.73.

I put together a spreadsheet to show RPM changes for different scenarios. It uses 6L80 ratios, assuming TC is locked. If you want this in excel to play with different ratios, PM me your email and I'll send it.

Opinions wanted!

Geotrash · May 24, 2024

I tow a 7500lb camper with the 3.42 and the 6L80, and am completely happy. I run at 65 mph in 4th most of the time when towing because Nick Transmissions says that's the strongest path through that transmission. I'm turning about 2800 RPM with rarely ever a downshift needed, unless on a steep incline. If I had 3.73's I'd be over 3000 in 4th, with zero practical benefit for power. This is with a modified 6.2L though, so perhaps with the 5.3 that extra 200 rpm would be beneficial.

Marky Dissod · May 24, 2024

Since you mentioned 5.3L, and hills, I'd go 3.73 - cheaper than 3.42 & a cam swap.
Enjoy these calculators, everybody.

2125RpM, 265/70R17 (31.60"), 3.73, 6th (0.67) = 80MpH
2125RpM = 80MpH = nowhere near 'screaming'
If highway RpMs are more than you'd prefer, 3.73 would let you use even taller tires.
3.73 would not severely penalize highway MpG, it'd just spend more time in 5th & 6th than it does now.
If you're not a leadfooted schmuck like myself, 3.73 might even lighten your foot.
3.73 would also improve metro / urban MpG, even before the tune.

Trey Hardy · May 24, 2024

DaveO9 said:
I know I want to change out the 3.08s in my '13, 5.3, 4wd Tahoe, just not sure if I want 3.42 or 3.73. Some factors I'm considering:

- I don't tow anything heavy yet, but my wife and I are considering a small travel trailer. Upper weight limit would be about 5K.
- Would like to stay with the 5.3, without major mods. (not opposed to a cam swap and related. Still have AFM valvetrain, disabled in tune.)
- Hills are a factor in the PNW. Just about any place you go with a trailer will involve significant grades and probably mountain passes.
- Don't want a screamer going down the freeway unloaded at 75 mph. Also don't want to cause big impacts to unloaded highway mpg. (But I'm pretty unclear as to how much effect several hundred rpm has. I've tried some experiments by shifting to 5th and then matching what my rpm would be with different gears, but I think I'm just seeing effects from small grade changes. The experiments did show me, however, that it's not a big difference in noise between 1600 rpm and 2000 rpm.
- rather than swapping R&P, I plan to just buy junkyard compete assemblies, front and rear. Most units I'm looking at have fewer miles than my current ones. But....
- I'd really like to do the upgrade to the 9.5"/14-bolt rear. This adds several hundred to the cost for the 3.42 ratio, but even more for the 3.73 since they never used that ratio with that assembly. I'd have to do a R&P swap if I want both 14 bolt and 3.73.

I put together a spreadsheet to show RPM changes for different scenarios. It uses 6L80 ratios, assuming TC is locked. If you want this in excel to play with different ratios, PM me your email and I'll send it.

Opinions wanted!

View attachment 429121

I’ve got 3.73s in both my truck and Tahoe and they have done fine towing wise and I’ve towed up to 10-15,000lbs
However I have larger tires and I want to switch to 4.10s or maybe even 4.56s in the future but the 3.73s get the job done for now

j91z28d1 · May 24, 2024

Geotrash said:
I tow a 7500lb camper with the 3.42 and the 6L80, and am completely happy. I run at 65 mph in 4th most of the time when towing because Nick Transmissions says that's the strongest path through that transmission. I'm turning about 2800 RPM with rarely ever a downshift needed, unless on a steep incline. If I had 3.73's I'd be over 3000 in 4th, with zero practical benefit for power. This is with a modified 6.2L though, so perhaps with the 5.3 that extra 200 rpm would be beneficial.

so you can get the axles used with 3.42 but if you want 3.73 you gotta buy that alxes and then have gears installed too?

this seems like the thing to do. especially because of how much more complicated the 3.73 is. maybe put the axles in as you get them and if you feel like you need more, have the gears done then?

DaveO9 · May 25, 2024

j91z28d1 said:
so you can get the axles used with 3.42 but if you want 3.73 you gotta buy that alxes and then have gears installed too?

this seems like the thing to do. especially because of how much more complicated the 3.73 is. maybe put the axles in as you get them and if you feel like you need more, have the gears done then?

For the front, I can get a junkyard unit with either 3.42 or 3.73, no problem. But for the rear, since I want to upgrade to the 9.5" ring gear/14-bolt, I would have to do a ring and pinion swap if I wanted 3.73. That 14 bolt was just offered in RWD escalades and denalis (and Avalanche?) with the 6.2, and they never offered 3.73. (4.10 was an option, but those seem to be really hard to find, and that's more gearing than I want)

I've thought about the try before I buy option: buy the 3.42 rear end, install it, and wait to buy the front after I've driven around a bit. (just got to make sure I disable 4wd

) I'm still checking to see how much the r&p swap will cost me. I've come to the realization that the swap is a bit out of my comfort zone and tool availability.

Thanks to everyone that chimed in with opinions and experiences. Definitely taking them into account.

Next question: was g80 gov-lock a common option for the trucks with the 14-bolt? The used parts listings I've been looking at don't mention if they have G80. I definitely want to keep that option.

mikeyss · May 25, 2024

My 09 Tahoe has the AXN 14 bolt 3.42 G80 axle from the factory. I have almost 190k miles on it and it is still working perfectly.

DaveO9 · May 26, 2024

Interesting! I didn’t know the 14-bolt was an option in GM 900 tahoes! Thanks for sharing

mikeyss said:
My 09 Tahoe has the AXN 14 bolt 3.42 G80 axle from the factory. I have almost 190k miles on it and it is still working perfectly.

mikeyss · May 26, 2024

DaveO9 said:
Interesting! I didn’t know the 14-bolt was an option in GM 900 tahoes! Thanks for sharing

It was only a 1 year option and it also came with a L9H

buckwild27 · May 26, 2024

if you're going to be pulling something heavy on a regular basis I would go 3.73's and get a good tune on engine and transmission. My 2013 Sierra has 3.73's since shortly after I bought it and I pulled a 28' camper or a 28' enclosed trailer with it all the time in 6th gear without the tow/haul button the tune eliminated the need to use it. And it would drag the trailers at 70-75mph no problem. Gas mileage was crap at 75 I got at best 5-7mpg and at 65 I could get 7-10mpg depending on terrain. Bought the truck new it has 243,000 miles on it now no engine or transmission issue to this day.

Need opinions: 3.42 vs 3.73 GR

DaveO9

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Marky Dissod

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Trey Hardy

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