What gear/axle combo to go with?

[87Skier]

Need help deciding what I should do here. I am changing my 72 GMC Jimmy 4x4 to overdrive transmission, with NP205, 12 bolt rear, Dana44 front and 33" BFG AT.

Currently, it has 3.07 gears. I'm going to change them when I get the new transmission. For the rear, I'm pretty sure that I am going with 3.73, and a limited slip Detroit Truetrac.

The front carrier will accept up to 3.73, but it is open diff.

It gets driven on the street most of the time, but it does see some off roading. Nothing extreme though since it is an old truck and I would rather not bash it into rocks or trees.

So here is my question, should I go with 3.73's and leave the front open? Or should I spend the extra and go limited slip front and rear with 3.73 or 4.11? I'm limited my gearing options to these since they should be good on the highway.

What would you guys do?

[72MARIO]

Personally I would go 4:11 with od and 33s. Nothing worse than a lugging 4th gear .

[72MARIO]

Randys ring and pinion has a great rpm calculator if you want to play with different gear ratios

[bilfman]

I have done just this with the same tire. I went with 3.73 and really wish I went 4.10. Having done it and lived it , my 100% recommendation would be 4.10 hands down. That extra 10% gearing would make a world of difference in my rig , both highway and bottom end.

[b454rat]

I say 4;10s as well. Since your gonna be in the front anyways, might as well put a posi. IIRC, Honda Passports and Izusu Rodeos 1998 and older will work. They are a Tracklok, an OK unit, good for what you want to do. If you want something little stronger, get a Powerlock. As far as posi's go, they are the strongest. I finally found one on c-list after looking off and on for 20 years....

[snj8198]

with OD, another vote for 4.11's, especially with a small block.

[87Skier]

I'm actually surprised it has been a unanimous vote for 4.10s.

Would the recommendations still stand with a sb 400? I have one I've been thinking about building that should make good torque. I think the way it is set up now is decent, so I having trouble imagining 3.73s let alone 4.10s.

[special-K]

I would say you are right on the line with 33s and 3.73. It will work, but you will gain less pull than 4.10s while not eliminating all "too tall" in O/D situations. The O/D "allows" for 4.10s to be good enough for hiway while you've gained new power for your bottom end pleasure. The rigs I've had with O/D all had 4.10s with 33-35" tires. Some say 35" is too tall for 4.10s & O/D. My trucks were diesel and it worked well. What most experience with that combo is transmission "searching" between Drive and O/D with gas motors. I think that's where you would be with 3.73 & 33s. That's an excellent ratio with factory size tires. Check out that chart at Randy's for hard numbers that don't lie

[ryanroo]

ive had a non OD truck with 373s and an OD truck with 4.10 both 350 sbcs both with 33"s and both 72 chevys. i would run the 4.10 as well. its a very nice gear package for going down the road and leaves you with plenty of reduction in the slower gears.

[A1971Blazer]

I'm doing an LS1/4L60E swap into my '71 Blazer and 33" tires.......4.10 was my long researched choice.....You want the engine in it's "sweet spot" for OD cruising......I'm sticking in a Truetrac while I'm at it too.

Also you can get a "thick" gear set that will run on your existing carrier....although the open carriers are relatively cheap.

[El Dorado Jim]

4:10 is what I run in all my trucks, great for towing, pulling,with the OD in your rig you will have the best of both world's, offroad and highway, I don't have OD, but am planning on a gear vendor in my K20suburban, and thinking about nv4500 conversion in my '72 K20

[Bigdav160]

I'm running 33" w/3.73 gears and a 700R4 behind a warmed up 350. Not alot of miles but I've driven this combo many times before and I'm fine with it.
edit to add, e-locker up front and a G80 in the 14 bolt out back.

My daily has 33" w/3.55:1 gears with a manual NV5600 A little more torque from the engine though.

[A1971Blazer]

Originally Posted by Bigdav160

I'm running 33" w/3.73 gears and a 700R4 behind a warmed up 350. Not alot of miles but I've driven this combo many times before and I'm fine with it.
edit to add, e-locker up front and a G80 in the 14 bolt out back.

My daily has 33" w/3.55:1 gears with a manual NV5600 A little more torque from the engine though.

How's the fuel mileage.......?

[BigRed76]

I would run the 4.10's. I had a truck with a 350 Tbi, 33"s, 3.73's, and 700R4, and in 4th gear it was a bit sluggish until speeds were high enough to get the rpms in the 2200-2500 range. I no longer have the truck but always wished I had gone with 4.10's.

On my GMC I have now, it's got 33's, 4.10's and no OD, and with the rpms it runs at highway speeds OD would be great for it.

[87Skier]

I guess 4.10s it is!

Have to ask this, but what if I went up to 35" tires. Would 4.10s still be enough gearing?

[andrewmp6]

If its a daily driver 35s and 4.10 would be ok,Off road and you want more power 4.65 or 4.88 would be better.

[duallyjams]

Originally Posted by 87Skier

I'm actually surprised it has been a unanimous vote for 4.10s.

Would the recommendations still stand with a sb 400? I have one I've been thinking about building that should make good torque. I think the way it is set up now is decent, so I having trouble imagining 3.73s let alone 4.10s.

My favorite 4x4 that I had was a 400sb with a sm465 and 4:10 and 33 tires daily driver, sled pull, mud bogg and off road every weekend.