Rear-end broke: regear or same ratio?

[Keitha]

1991 Suburban 3/4 ton, 14 bolt semi-float rear-end, mild-ish 383", 285/75/16 tires (32.84"), stock 3.73 gears, 4L80e trans

My rear pinion sheered a few teeth. It is currently in a shop being rebuilt. I am going with a Truetrac diff and deciding on the gear ratio.
In SoCal, I freeway speeds are typically 75+ (80 at times). I don't go off-road much, but the occasional trails - nothing hard core. I do tow a 5,000lb boat occasionally.

Just looking for guidance: While the rear is being worked on, do I change to a 4.10 gear ratio - then I will have to re-gear the front end ($1,000+ gears, seals, etc) to match, or do I live with the 3.73? I don't plan to run larger tires - 33" tops if anything. Will I notice much difference between the 3.73 and the 4.10? I have always felt like it could use a bit more, but just don't know if the cost is justified.

Carnage shot:

 

[Blue85]

3.73-->4.10 is a 10% switch, so you will notice it, but it won't be a huge difference. Like if you cruise at 2000RPM now, it will go up to 2200. 1st gear in a 4L80 isn't very good, but the OD is normal. 4.56 would be better for performance and a 22% shift is definitely night and day. It's up to you what the priorities are between performance or a little more cruising comfort/economy.

How did you break it? Did it develop a loose pinion?

 

[tarussell]

Agreed about only uping the gear by 10% might leave you questioning was it worth the money spent - 4.56’s will work great in a heavy Burb with the O/D of the 4L80.

 

[Keitha]

Thanks for the response. I prioritize highway cruise/economy if I had to be pegged down on this.

I recently changed my fluid and noticed a couple very small chunks from one of the spider gears. I cleaned it out, closed it up, then noticed a lead from the pinion seal. I replaced the seal and thought to myself that the pinion nut was not very torqued. I replaced it exactly as it came off (same number of turns etc).
I asked the shop if I screwed that up and he said, No, the pinion is not lose at all. Likely another chunk from the spider gear just got between the pinion and ring.

With the 4.56, I am worried it will be too high RPM cruising at 75 mph.
How would economy be effected going to a 4.56 at highway speeds?

 

[Wes Harden]

285 75r16 + 4.56 equal 2650 rpm at 75 mph. I went with 4.88 and 285 7516 for quite awhile that work out to 2840ish.

For 10% don't think it's work the $$, if you were to it yourself then maybe.

 

[warrpath4x4]

You can use this link to play around with different gear ratios, tire size, and transmissions

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.

spicerparts.com

 

[tarussell]

In some cases the added RPM’s while at highway speeds can help overcome a ton of wind resistance in that rolling brick and allow less throttle effort ultimately gaining better economy and performance at the same time.

Like said above - play around with the calculations and see just how little the RPM’s will change.

 

[obijuank5]

Definitely regear it. I’d put 4.56 if it were me. Well it was me in my 91 actually.

 

[sweetk30]

You can use this link to play around with different gear ratios, tire size, and transmissions

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.

spicerparts.com

Grimmjeepper is way better and easy .

 

[sweetk30]

I have a 15k gvw med duty truck with the 8.1/nv4500/4.63 gear on 33" tires and haul loads and tow 8-10k loads . 75mph 2,500rpm is just fine .

Our old trucks before overdrive saw even more rpm and less highway speed . They survived .

And 3.73 to 4.10 you wont fell much . Always do a min of 2 ratio to notice it .

With the tires and gears you need to do the modified DRAC unit under the dash to correct the speedo . I know a guy on this page who does them for members .

 

[Big Ray]

Thanks for the response. I prioritize highway cruise/economy if I had to be pegged down on this.

I recently changed my fluid and noticed a couple very small chunks from one of the spider gears. I cleaned it out, closed it up, then noticed a lead from the pinion seal. I replaced the seal and thought to myself that the pinion nut was not very torqued. I replaced it exactly as it came off (same number of turns etc).
I asked the shop if I screwed that up and he said, No, the pinion is not lose at all. Likely another chunk from the spider gear just got between the pinion and ring.

With the 4.56, I am worried it will be too high RPM cruising at 75 mph.
How would economy be effected going to a 4.56 at highway speeds?

Gear Ratio Calculator

www.grimmjeeper.com

I get hours of entertainment playing with this.

 

[Keitha]

Gear Ratio Calculator

www.grimmjeeper.com

I get hours of entertainment playing with this.

I spent way too much time on this today. Thanks for the link.
Great advice here and while I do think 4.10 are probably the best, I agree that the gain is so marginal it's just not worth the money. 4.56's would be perfect Except for the fact that the flow of traffic in SoCal is 80+ mph

Decision made - but I don't feel great about it: I am staying with the 3.73 for a few reasons
- The shop said there supplier cant get any 4.56 gears till late in the week at best - maybe longer.
- I just don't want to spend the extra money right now on the front end
- I'm worried that at 80+ mph, the 4.56 will be revving too high. the calculator has it at 2800 rpm at 80 mph.
- I know what to expect with the 3.73. It's not bad, I just know it could be better.

My daily driver is down right now as well and I need my Suburban running, so I don't want to wait for the gears. My daily driver may be $2,000 to get it going. It's a 2005 Mercedes S55. Great car, but the hydraulic suspension pump is leaking. I don't want to flush good money after bad trying to keep repairing the system, so I may convert it to coil-overs.

 

[Big Ray]

I have a set of 1 tons I picked up with 4.88 gears. Going to step up to 35's or 37's.

Personal note, i joined this site in '09. Finally have 95% of what I need to build my dream rig!
Of course, I'm an old broke down old fock now. But the dream keeps me going!
Lol

P.s., I have an 86 and a 90 Suburbans

Still miss my K5. Still spit on the ground when I say "that bitch" when I talk about that wreck.

Rant off lol.
Sorry for hijacking...

 

[Blue85]

1991 Suburban 3/4 ton,

I prioritize highway cruise/economy

Pick one?

 

[Keitha]

Pick one?

Ha! It's purely relative - Not looking to compare to any modern rig, just want it to have good highway manners. I don't think I would ever relax cruising on the highway with the motor turning 2800 - I would always be looking for that next gear shift.


Big Ray, Get that dream rig rolling!

 

[Keitha]

Would this make a funny noise when driving?

 

[Blue85]

Mint!

 

[imiceman44]

Ha! It's purely relative - Not looking to compare to any modern rig, just want it to have good highway manners. I don't think I would ever relax cruising on the highway with the motor turning 2800 - I would always be looking for that next gear shift.


Big Ray, Get that dream rig rolling!

I know you already made the decision but I was in your shoes and I ended up upgrading by swapping axles, first I got the rear so I could drive it, then I found the front a few months later.
Cost was less than $1000 for both and 4.10 was perfect for CA traffic.
I did step up to a 14bff as well in the process so there's that

 

[Keitha]

I know you already made the decision but I was in your shoes and I ended up upgrading by swapping axles, first I got the rear so I could drive it, then I found the front a few months later.
Cost was less than $1000 for both and 4.10 was perfect for CA traffic.
I did step up to a 14bff as well in the process so there's that

Yah, I had to get it back on the road as my daily driver is down at the same time. I like your advice - I may start looking for a front. I do feel like the 4.10 would be the right call.
What size tires are you running?

 

[imiceman44]

Yah, I had to get it back on the road as my daily driver is down at the same time. I like your advice - I may start looking for a front. I do feel like the 4.10 would be the right call.
What size tires are you running?

33 was for 12 years. Started with 3.73 for 7 years and 5 on the 4.10
Now it's sitting in the backyard waiting for a remake:
8.1/nv4500/4.56/37"