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37s stock 3 speed and Dana 20... What gears?

76broncoorange

Jr. Member
Joined
Dec 14, 2010
Messages
186
Have a stock 3 speed stick, stock Dana 20... 35 spline rear... Front has chromoly axles... About to regear.... I was wondering what I should go with... Max speed is 55mph... Mostly a mud truck/wheeler/toy I drive to the wheeling spots

I have 4.56s on the bench but thinking i should go to 5.13/5.14s....
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
35,634
I always think a great non-OD mix is 33s and 4.11 gears. Scale that up to 37s and you need ~4.61 gearing. So 4.56 would be the least gear I would run. You also have a lot more rolling and wind drag. And with plans of maxing out at 55MPH, a bit more gear. So 4.88~5.13 range is looking very good.
 

Apogee

Contributor
Bronco Guru
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Nov 26, 2005
Messages
6,194
Your clutch would appreciate the 5.13's, and since your max speed is 55, that would work reasonably well.

I'm setting up my 4.88 gears now for a 351W/NV4500/Atlas 3.8/37" rig, but it needs to do 75 MPH and has the added torque from the 351W. I could have just as easily gone 5.13, but didn't for some reason...we'll see if I regret that, but it's a small difference either way.
 

4x4man514

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Aug 28, 2003
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I had 37s with 4:56s, a zf and a wore out 302. It felt good. plenty of grunt and all the top end I wanted. I wouldn't change anything.
 

DanHall

Jr. Member
Joined
Feb 18, 2013
Messages
234
Loc.
Pueblo, CO
I have been running 35s and 4.88 and it that gives me plenty of oomph with a top speed around 65. 37s and 5.13 would be close to the same.
 

Nobody

Bronco Guru
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Feb 15, 2005
Messages
1,215
Loc.
Stanwood
Axle gears really have more to do with setting cruising RPM than offroad performance. Consider the actual numbers

3 speed x dana 20(t-shift) x axle ratio = compound low
2.99 x 2.46 x 4.56 = 33.54

Now lets say you go ahead a spend a bunch of money and gear it at 5.13.
2.99 x 2.46 x 5.13 = 37.73

Lets say you get crazy and drop in a 4:1 atlas transfercase
2.99 x 4 x 4.56 = 54.54

Ok, that's getting better. But you've still got some cash burning a hole in your pocket, so you gear it out at 5.13 too.
2.99 x 4 x 5.13 = 61.35

Now looks what happens if you drop in a ZF.
5.72 x 2.46 x 4.56 = 64.16

Not bad at all. But you say you'll primarily be on the trail, so you really don't care about that ZF Overdrive. How about an NP435?

6.69 x 2.46 x 4.56 = 75.05

See where your problem is?

The NP435 or even T-18 are very cheap ways to get great bulletproof gearing. You can pick these transmissions up for $50-200. Other than some tunnel mods, they pretty much bolt right up with a transfercase adapter. No driveline mods required.

I run the NP435 and 4.56's with 36's and I'm very happy with that combination.

30a6dr8.jpg
 
Last edited:
OP
OP
7

76broncoorange

Jr. Member
Joined
Dec 14, 2010
Messages
186
I have a np 435 sitting in the garage and it's the 2wd version just need to save for the adapter..... So are you saying stay with 4.56 and spend the money on the np435 swap?
 

Nobody

Bronco Guru
Joined
Feb 15, 2005
Messages
1,215
Loc.
Stanwood
The NP435 will give the lowest crawl ratio for the least amount of money. I.e. Best bang for the buck.
 

Prizefighter

Bronco Guru
Joined
Feb 24, 2010
Messages
1,192
The 4:56, 4:88, and 5:13 are all in the optimal range according to the chart. I have a 302/3 speed manual/D20 and Q78 (36") tires on my street/trail rig. It's good but I'm a little curious what 4:88s would feel like since my start seems a little slow.
 

sykanr0ng

Bronco Guru
Joined
Aug 11, 2014
Messages
5,363
With the ZF5 2nd, 3rd and 4th are pretty close to the ratios of the stock three speed.

Plus an overdrive to go to the trail.

Plus a 'granny low' for on the trail.
 
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