Gear ratio concerns

[Grey owl]

My 69 Dana 30 gear set was destroyed. The new gear set is 3:54 the old was 3:50. The rear is 3:50 . I was told if I try run it in 4X4 hubs locked it will cause catastrophic drive line failure. The problem is I cannot find a 3:54 or 3:55 gear set for a 9 inch ford. What are my options and how closely matched do the difference have to be.
Thanks

 

[Glass Pony]

My 69 Dana 30 gear set was destroyed. The new gear set is 3:54 the old was 3:50. The rear is 3:50 . I was told if I try run it in 4X4 hubs locked it will cause catastrophic drive line failure. The problem is I cannot find a 3:54 or 3:55 gear set for a 9 inch ford. What are my options and how closely matched do the difference have to be.
Thanks

The 3:54 will be fine. My bronco is 4:11 rear 4:09 front. Are you sure the old gears were 3:50?

 

[Timmy390]

3.54 and 3.50 was stock as I recall

I too run 4.09 and 4.11

Tim

 

[jamesroney]

EVERY 1971+ "3.50" Early Bronco with a Dana 44 and 9 inch ran:
46/13= 3.54 front
35/10= 3.50 rear.

The "3.50" Early Bronco with a Dana 30 and 9 inch ran:
35/10= 3.50 front

The most common replacement gears for the Dana 30 are 46/13 = 3.54. They are a hunting pattern with superior metallurgy. Everything about them is better.

The difference between 3.50 and 3.54 is about 1%. Whoever told you that it would be a problem was assuming that the difference was much bigger. It's not. Ford did it. You are fine.

 

[DirtDonk]

What they all said.
But the person that warned you about damage was probably just taking factory recommendations, adding the horror stories of people with truly “mismatched“ gear sets doing damage, and mixed their own imaginations and forming the wrong conclusions.
The minimal difference that we’re talking about here, is not normally going to cause a major damage issue, but it is a cause of mild binding.
And is why that even Ford recommends that you don’t run around locked in four-wheel-drive on hard pack surfaces.
It’s a balance.
On the one hand, they recommend locking the hubs every now and then and drive around to keep things spinning and lubed up.
But you don’t put your t-case in four-wheel-drive and lock the hubs on the street. Not for very far anyway.
However, you’re not likely to actually do catastrophic damage to something with your quite normal slight mismatch. It’s when somebody changes their rear ratio and doesn’t bother to change the front, or vice versa that things start to get ugly.
In other words, you can’t run a 354 and a 411 at the same time!

 

[Broncobowsher]

You (should) only use 4WD in low traction conditions. Where 2WD does not have enough traction. If 2WD has enough traction, just use 2WD.
So 4WD when tires can slip. Different ratios, you get the tiniest amount of slip. That doesn't matter since the traction is so low you need the 4WD.
That's just going in a straight line. Make a turn. The front axle travels further than the rear. That is why you get that binding feel when turning in 4WD. You just power through and get a little tire slip in the process. Tire slip is just part of normal 4WD usage.

Now having ratios close is good. It's less slip. But there will always be slip. Matched ratios or not.

 

[Grey owl]

The 3:54 will be fine. My bronco is 4:11 rear 4:09 front. Are you sure the old gears were 3:50?

The diff tag said 3:50