"My Bronco desperately needs lower t.case gears..."
Have U tried it to know (ie w/the NV)?

Have U tried the online calculators?
there's a bunch (some better than others). I would hafta use em due to my math. same w/the Taxes (I always owe them or they me, less I use my accountant).
kouwell! Bob the engineer! I shoulda known from the precision of those welds in yer pic!


"3.15 : 1 is a replacement crawler gear for the Dana 20"
is that the $800 I mentioned earlier "from the typical vendors"? I don't C it right now in JBG's, WH's, Tom's, or Duff's now... may B these catalogues are too new? As I recall it ended up more extreme (4:1). I think I'd rather a locker for that kinda money (if I had it!)...

 

I've only had one semi-serious off-road trip with the Bronco. With the stock trans and case it just doesn't go slow enough. I had to slip the clutch way too much to crawl over rocks, otherwise I was banging off them way too hard or stalling the engine way too often. The Jeep 258 was barely acceptable to me (I always wanted lower gears, but I could go where I wanted without slipping the clutch and without stalling the engine too much). The Bronco has the same crawl ratio my Jeep did, but the 302 just doesn't idle down and lug like the 258. I'm happy enough with the engine otherwise, but it needs lower gears than I have.

The NV3550 has a 4:1 1st gear compared to the 3.4:1 1st of the RAT 3-speed. That only drops my crawl ratio from 34.4:1 to 40:1. Looking at it another way, it drops my speed at 1000 rpm from 2.9 mph to 2.4 mph. That doesn't seem like enough of a difference, so I certainly don't want to bet the $459 cost of an NV3550-Dana 20 adapter that I'll be happy with it. I'd rather that $459 be the seed money toward an Atlas and just do it once. With the NV3550 and Atlas 4.3 at 1000 rpm I'll be going 1.4 mph.


Haven't tried any so I can't speak to them.

I'm an engineer, I'm certainly no welder! I can usually melt metal with a MIG well enough to keep it from falling apart. But I'm no artist.

$800 for aftermarket crawler gears sounds about right, but I haven't looked that hard. I know there are 4:1 crawler gears for a lot of cases, but I'm pretty sure the crawler gears for a Dana 20 are 3.15. Add the $800 for the gears to the $460 to adapt the NV3550 to the Dana 20 and you're getting close enough to an Atlas that I decided not to settle for 3.15 gears.

Obviously with a project like this there are a thousand different possible combinations. I certainly didn't land on the cheapest, or the best. But I think I landed where I want to be. If someone else is making the decisions, paying the money and then living with the results they may well end up somewhere else. And that's fine.

Besides, I've already got a Detroit in the rear!

 

"...Looking at it another way, it drops my speed at 1000 rpm from 2.9 mph to 2.4 mph..."

OK! now that's more helpful to me than the "X:1 verses Y:1" I often see on these forums. It's alot more useable 4 me. How do you 'vert those ratios to mph, the way you've done? I could try that for more realistic personal info than I got now...

"... I landed where I want to be..."
AND -
that's a Great thing!

 

The "simple" answer is engine speed (rpm) x tire diameter (inches) / overall gear ratio / 336 = vehicle speed (mph)

Overall gear ratio is all of your gears multiplied by each other. So if I'm interested in how fast I'm going in 1st gear, low range, my overall gear ratio is 3.41 (1st gear ratio) x 2.46 (transfer case low range) x 4.1 (axle ratio) = 34.4.


(Edit: Ran out of time last night, adding more content)

So at 1000 rpm in 1st gear low range I'm going: 1000 rpm x 33 inches / 34.4 / 336 = 2.9 mph

Once the NV2550/Atlas is in my overall gear ratio in low-low will be 4.01 x 4.3 x 4.1 = 70.7.
So at 1000 rpm in low-low I'll be going 1000 x 33 / 70.7 / 336 = 1.4 mph

Or in 5th gear, low range my overall gear ratio will be 0.78 x 4.3 x 4.1 = 13.8 and at 2500 rpm I'll be going 17.8 mph. So 1.4 to 18 mph will be my useable speed range in low range.

It's also helpful to be able to figure out what your engine speed is at any given vehicle speed in any particular gear. A little algebra rearranges the equation to be:
vehicle speed (mph) x overall gear ratio x 336 / tire diameter (inches) = engine speed (rpm)

So as an example, with my current setup my overall gear ratio in high gear, high range is:
1 (3rd gear ratio) x 1 (transfer case high range) x 4.1 (axles) = 4.1

So 70 mph in high gear is: 70 (mph) x 4.1 x 336 / 33 = 2922 rpm

After the NV3550/Atlas my overall gear ratio in 5th/high will be 0.78 x 1 x 4.1 = 3.20
And my engine speed at 70 mph will be: 70 x 3.20 x 336 / 33 = 2281 rpm

 

For me, a systems analyst (human systems and buildings; 2 very different systems) - the formulas always look just as they should. I.e. U read them and they say what is intended/give the answer sought.

But heck, I could never figure them out/design them myself. I need metrics to analyst building performance, use the blower door and advise contractors on new construction, retrofits, and energy efficient or renewable energy design. On the other hand, human systems (families, schools, ship's crew out at sea, municipalities, businesses, etc, etc) go by a whole nother format ('self organizing')...

So I will thank you for these 2 useful ones and pocket them w/the hundreds of others.

VEHICLE SPEED (MPH)=(RPM) X TIRE DIAMETER (INCHES) / OVERALL GEAR RATIO / 336

ENGINE SPEED (RPM)=(MPH) X OVERALL GEAR RATIO X 336 / TIRE DIAMETER (INCHES)

Hummm, hope I never have an unknown in the right half of the equation. Get a lill tricky for this pea brain. Also I see the magic divisor, 336... may B the Golden Mean for the auto...

 

Update. I ended up going with an NV3550 5 speed and an Atlas 2 with 4.3:1 low range (4,10:1 axles and 33" tires behind a 302 in a '71 Bronco). Finally got to try it off road and I love it! 5th gear low range is high enough for almost all trail riding, so there's not too much hunting between low and high range, that was my biggest concern. And 1st gear low range makes the rocks a lot easier! I used to have to run the engine at least at 1200 rpm to keep it from stalling too much. That was 3.4 mph in low-low. Now it will idle at 800 rpm, giving me only 1.1 mph with the new gears. That means I'm hitting the rocks with about 1/10th the energy I used to. Very noticeable!

 

Obviously not what you're looking for, but this would be stupid low.
Unimog's U500Is - Ultimate Road Test - Truck Trend

 

Time for another update. I took the Bronco on another fourwheeling trip (pictures are in the "4x4 PICTURES" thread in this forum). Man I love this gearing! On the trails I almost never shift out of low range. With the twin stick it's easy to shift into 2-low (and with a Detroit in the rear 2-low works pretty well), so I do that if I'm going very far on anything relatively smooth. And I end up in 5th gear pretty often like that, but the 15 - 18 mph I can easily do in 5th gear - low range is usually fine for smooth trails.

And in the rocks it ROCKS! 1st gear low range and you don't have to touch the pedals hardly at all. And if it does stall, hit the key without touching the clutch and it almost always fires right up and just keeps chugging along. And it's going so slow that when you do hit the diff into a rock it's a MUCH less hard hit. Hitting rocks 1/10 as hard makes a BIG difference!

After we got home from the trip I was looking at fourwheeling video of my old Jeep and my Bronco before the Atlas t.case. Compared to what it looks like now, I could hardly believe how fast I used to have to "crawl!"

As I said at the start of this thread, I don't do mud or hill climbs where tire speed is a big factor. If I did I might feel differently. But for rock crawling and general trsail riding I couldn't be happier with my gearing options. I still think an Atlas 4 speed would be cool, but I'm sure glad I still have that extra $1,000!

 

I have a friend trying to use

6.32 nv4500 or 6.55 sm465
2 np203
4 Toyo t case
5.38 gm 10 and 12 bolt
=
272 or 282
I keep telling him it's dumb he doesn't listen

 

It might be kind of nice to have the option of either the 2:1 or the 4:1 t.case. To tell the truth I still think I'd like to have the Atlas 4 speed (that would give me something like 192:1 crawl if I used low-low-low), but I just decided it wasn't worth the money for me. But if I did a lot of hills or mud I might feel differently about only having the 4.3:1 low range.

So the thing I'd be most concerned about your friends plan is the 10 bolt and 12 bolt axles. I sure wouldn't put much money into those if I was putting that kind of gearing ahead of them. I'd want something a lot stronger.

In my case the 9" and Dana 44 might be considered weak links, but I do have stronger than stock axle shafts in both, and I don't drive with a lot of skinny pedal (no hill climbs or mud, remember?).

If your friend isn't planning on romping on it the axles might hold up fine, but in that case he probably doesn't need the 2:1. Still, it's his truck and his money, so I won't stand in his way.

 

I'd take the ZF over the NV 4500, but for my needs/cash the 3550 is the way 2 go. D44 (got it, seek the Hi-Pinion - 'full clip' shortened) and 9 inch are just right for the needs here. A 203 transfer would B nice but probably too heavy/over kill.
All mods all ways depend on the application and compatibly w/the rest of the SYSTEM.

 

It's all chevy stuff going into a Toyota he has it all already including both transmissions he just needs to put it together he's leaning towards the nv4500 because it shifts easier

 

Oh, OK, cool.
I thought he wuz goin "Cold Duck" or whatever they call it when they put 2 transmissions together (or actually the frnt range box & a 2nd trany).

 

Both transfer cases my bad

 

So he's just doing a doubler behind a manual trans?? What's wrong with that