Axel gears?

[NORK2BG]

Building a Bronco plan on swapping a 98 or newer explorer motor with the auto trans (4R70). Will be running a Dana 20 with a heavy duty output shaft and 37 inch tires mostly driven on the street. Leaning towards 4.88 gears to keep the contact patch greater than 5.38’s. Other Bronco owners that are running this set up. What gear ratios do you recommend?

 

[Broncobowsher]

Having run the 4R70W and 4.88s. Sort of wrote the book (the 20+ year old tech article before the full Explorer swap was real).
I will say 5.38s. Here is why...
Street use, you are not running it hard enough for the small tooth to be an issue.
The stock 5.0 Explorer you said you are planning on running doesn't make enough power to hurt the small pinion. There are people putting exponential levels more power through 5.38s and not hurting them. Small pinion is not an issue for you.
37" tires are tall, lots of drag, rolling and wind. You need to help that little 302 to stay in the happy spot while pushing all that drag.
A 5.38 gear will get you 2000 RPM at 60 MPH, 2500 at 75, and if you try and run 90 it is only turning 3000 RPM. Really isn't that deep of a gear. I question of there is enough torque at 3000 to actually push that rig at 90, but you really shouldn't be doing that anyway. 75 is fairly realistic, and 2500 RPM puts the 302 in the power band where it might actually be able to push that wind without being on the floor.

 

[Yeller]

Second 5.38’s. Also add a “Daytona” pinion support, it will keep that running happily for a long time. With 5.38’s the pinion is spinning very fast and the stock small pinion bearings literally melt.

 

[NORK2BG]

That’s what I needed to know. Thank you

 

[DirtDonk]

The descending pinion size for lower gears is more for the front Dana differential than it is for the rear Ford 9 inch. Correct?
Don’t remember it being a thing.

 

[Broncobowsher]

It is both. The ring gear is the same diameter regardless of ratio. The pinion is smaller than the ring by a factor of the gear ratio. A 9" with a 3.00 ratio has a 3" ring gear. Well with the tooth mesh and the bevel of the gear, not really. If it were smooth metal on smooth metal, the contact point would be 1/3. The Dana does have a smaller ring gear to start with, so that makes a smaller pinion. The 9" is a bit odd in they run a very long tooth (radial distance) on the pinion. I look at them and think they have 50% more tooth than a Dana. It is also one of the reasons it is hard to get good diff options in the 9", the space to put spiders, locking parts, etc is pretty tight.