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Good rear gear ratio for what I'm running?

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cookieman77

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New here and need info for my blazer.

It's a ye olde 84 with a th400 and 454 I swapped in. It's got the 12 bolt open rear and np203 and I want to swap to a lsd and maybe change the gear ratio. Gonna be running 35s and need to know what a good and balanced gear ratio is for cruising at highway speeds (~80 mph) while still having some get up and go. Will also be doing some off-roading here and there.

Recommendations for lsd units would be appreciated as well. I'm still in school so the cheaper the better (as long as it's not horrendous quality obviously lmao)

I'm sure there's a thread somewhere but I couldn't find one so I figured I'd make a personalized one
 
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Probably 4.10’s. I’m inclined to say 4.56’s but running 80 mph on the interstate without overdrive it might be a bit much. Conversely, 3.73’s are a little high for off roading with 35’s and could generate some heat in the transmission.

I don’t have any LSD input on the 12 bolt as I’m not familiar with what’s available.
 
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Do you know your current ratio? Or the rpm at your desired speed?
Here's a calculator, it's the one I use. You can enter all your variables to see what will change. I would swap the transfer case to a part time unit.

 
Man with a 454, Id say start thinking about a 14bff with the gears in it already.
A new set of gears, setup kit, and some kind of traction device your going to be into for several hundred, if you do the work.
 
4:10 is a common ratio in 14ff or aam 10.5 axles, plus they handle torque much better, and have lots of aftermarket support. Only down side is 8 lug, and loss of ground clearance
 
Decide if you want power offroad, or economy on the highway. Then you can use a chart like this to get an idea.

Gear Ratio / Tire Size Table | Interco Tire

This table shows the numeric change when you change your tire size.
www.intercotire.com www.intercotire.com

I would also lower your expectations for highway speeds with a 3-speed automatic. You will want to stay below 65 mph on the highways with an old rig like this.
 
Appreciate the help, all.

An od unit is in my future for sure; I just need a starting point until I can find one

A 14 bolt would be cool but I’d rather not have to find one or move to 8 lug.

Also the 203 that I have was converted to part time by the old owner so fortunately it can stay until I can replace it.

Will look into 4.10s probably, hopefully someone can come along to point me towards an lsd and such.
 
An Eaton or Auburn type LSD’s are mild enough ( they each come in various levels of spring loads controlling the aggressiveness ) and are fairly strong units.

Pic for reference.

6DE99C19-93DE-460A-9143-1FF662B252B2.png
 
Eaton, Detroit, Yukon, all good brands.
I'm leaning towards the Detroit Truetrac.
It's all gears, splits power where its needed.
 

you might want to look into a 6 lug Semi Float 14 bolt. depending on the condition of the 454 and your right foot, you may explode the 12 bolt; then again, it may last forever. and remember, if you change 1 axle ratio; you have to change the other axle as well.
also, there is a 12 bolt car axle and 12 bolt truck axle. make sure you are looking at truck axle parts.
 
I’d go 4.10, I have 4.56 with 35s and running down the intestate at 70 I’m turning about 3100 rpm, I don’t own anything with OD and between 3.54, 4.10, and 4.56 4.10 is the most well rounded
 
I like the Truetrac a bunch. I have one behind the big block in my '70 with 4:56 gears, with a 700R4 and 35s. It delivers power fairly evenly to both tires. Application of the brake will even help it pull better at crawling speeds, as it doesn't perform as a locker does in those situations.
Even on snow the Truetrac is good, because it doesn't let the back end walk sideways immediately, but smoothly transfers power.

I would suggest 4:10 gears without the OD, 4:56 if you are planning the overdrive soon and don't run the interstate much anyway.

I had 4:56 with 33's on my '70 for years. That was fun just and didn't bother me on the interstate because of OD. But my elevation is 4500' and goes up from there.

Oh, and my son has a Truetrac in his Dmax and his '01 2wd gas truck. Likes those units too.
 
Chickntrk said:
I’d go 4.10, I have 4.56 with 35s and running down the intestate at 70 I’m turning about 3100 rpm, I don’t own anything with OD and between 3.54, 4.10, and 4.56 4.10 is the most well rounded
Click to expand...
That with a torque converter or clutch?
 
fourwheelerjeff said:

you might want to look into a 6 lug Semi Float 14 bolt. depending on the condition of the 454 and your right foot, you may explode the 12 bolt; then again, it may last forever. and remember, if you change 1 axle ratio; you have to change the other axle as well.
also, there is a 12 bolt car axle and 12 bolt truck axle. make sure you are looking at truck axle parts.
Click to expand...
The main issue I’m having is that there’s only 1 scrap yard around here and whenever any square bodies show up, they get picked clean immediately so cheap axles aren’t easy to come by unfortunately. Then again it’s more of a future thing and I can run what I got for the time being till I have time to work and get money. The 454 is rebuilt and it’s got stock everything basically so I’m not too worried.
 
If you went 6 lug 14b you could source 1 from gmt800, maybe a late gmt400 and get rear disc brakes, with a good working parking brake. Perches and shock mounts will need to be moved. The G80 rpo in a 14 bolt is good to go for lsd.
 
cookieman77 said:
New here and need info for my blazer.

It's a ye olde 84 with a th400 and 454 I swapped in. It's got the 12 bolt open rear and np203 and I want to swap to a lsd and maybe change the gear ratio. Gonna be running 35s and need to know what a good and balanced gear ratio is for cruising at highway speeds (~80 mph) while still having some get up and go. Will also be doing some off-roading here and there.

Recommendations for lsd units would be appreciated as well. I'm still in school so the cheaper the better (as long as it's not horrendous quality obviously lmao)

I'm sure there's a thread somewhere but I couldn't find one so I figured I'd make a personalized one
Click to expand...
This calc has all the variables... lets you run 2 options side by side... Most run 3.73 with 35's, but at 80mph(!) you might need 3.55 or even 3.07/8 with a BBC. That is the range with a type III 12bolt rear end... 3.73 will give you about 80MPH at 3000rpm. 3.55 shows 2850 rpm, and 3.07 equals 2500rpm at 80.
 
I have 4.10 and it’s a good mix of power to highway ratio. But if I had a big block I could get away with 3.73 and still run a bit faster. I keep mine 65 and under because of how much it revs any faster on the highway. I’d check and see what you have in yours but 3.73 and 35s would be good for a big block.if you were more offroad I’d go 4.10 but for what speed you want to run it’s not great.
 
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