What Gears? 4R70W, 351W, 33" tires

[davisjstone]

Another 4R70W gearing question for those of you with experience. Bronco is not on the road yet, but I’m getting closer as I approach the 2 year mark.

What do ya’ll suggest as far as gearing?

Details:
Fully rebuilt 351W (0 miles)- full roller motor
Mild cam
ARF aluminum heads, Elderbrock RPM intake
Sniper 2 EFI
Rebuilt 4R70W w/rebuilt stock converter
Stock gearing 3.5
Planning to run 285/70/17 (32.7”).
USE: This will be a road only vehicle. Lots of on/off highway driving where I live (San Deigo)

Based on reading as many threads as I could find, it looks like 4.30, 4.56, or 4.88 are recommended. I’m trying to save money where possible and not overbuild, but also want to do things the right way once, as I seem to continually learn that lesson the hard way. Ugh.

Because of the 351W, I’m thinking I should move up to 31 spline rear axles, and at that point might as well do a Eaton truetrac rather than open carrier. Costs are snowballing quickly…

My plan was to just run as is (stock gearing) for a bit to see how things go, but I keep hearing with the current 3.50’s I will burn up the 4R70W as it hunts in/out of OD. I’d really hate to burn up a fresh trans. Also keep reading to just do the rear first and make sure its the right choice before doing the D44.

Which gears; 4.30, 4.56 , 4.88 or?

Bonus question: I’m learning/teaching myself as I go using this site, lots of research, and good ole YouTube videos. I’m sure I can get the 9” 3rd member removed along with axles etc, but I think the rear gear swap is beyond my skill level, certainly the D44 is. I’ve called around and prices for a swap are not easy to swallow. Anyone have a shop near Modesto, CA they recommend (bronco is in norcal currently)? Or do y’all recommend just buying a complete 3rd member online and slapping it in?

Numbers from GrimmJeeper. Left side is 4.56, right side is 4.88.

 

[rjrobin2002]

You will be happy with all 3, that 4r70w overdrive is nice.

I ran a stock explorer 5.0 with 4r70w and 4.10s with 35s for years and was happy with it . I was waiting for something in the axles to break and switch to 4.56 but no axles ever broke.

But I am in Louisiana with no long grades to climb while interstate driving.

 

[rjrobin2002]

I would buy a built 3rd member with Tru track and 4.56 gears and install it yourself if it was me and drive it a while.

Then save up for the front Dana 44 labor cost pain.

It is hard not to accidentally upgrade to 31 spline rear and heavy duty front axles while doing this stuff.

But I will say the 28 spline 9 " is almost impossible to break street driving, I had a bronco with a 347 stroker, 5 spd, 4.56 gears and Tru track and could do burnouts and drag race and never broke an axle. I was wild and in my 20s and gave that thing hell, it would pull a front tire off the ground drag racing.

 

[Broncobowsher]

I have experience with this combo. 351, 33s, 4R70W. Pretty close on the 351 as well, aluminum heads, ported Performer intake (before the days of the RPM line, but pretty much the same after the port job) My math said 4.56 was what I wanted. Found a deal on 4.88s.
While the 4.88s were good, the 4.56 would have been spot on. I was slightly overgeared with the 4.88s, but not enough to change.
This is in reference to the street use. drive it around town. Some decent highway trips as well. 3 round trips between Phoenix and Vegas. Very fun gear to drive around town, yet still great legs for the highway. Enough gear it wasn't hunting on the hills, it would pull most grades in OD. Great sweet spot.

 

[flousberg]

I have 351w, 4r70w, 35s, and 4.56. I am in Denver and going up mountains it would be better if I had 4.88. I think 4.56 would be perfect for 33s.

 

[Broncobowsher]

I have 351w, 4r70w, 35s, and 4.56. I am in Denver and going up mountains it would be better if I had 4.88. I think 4.56 would be perfect for 33s.

I will agree with that. One of my ideas for fixing the 4.88 overgearing was to step up to 35s. But packaging 35s and not killing all my suspension flex was a problem. I thought the combo could be fixed one of 3 ways. Smaller engine (not going to happen), taller tire (also not going to happen), or drop the gears to 4.56s (also not going to happen). So I just lived with it. Wasn't the end of the world, just a touch off perfect with no easy fix,

 

[flousberg]

I ended up with 4.56 because I changed gears when I had the three speed on the tree. Then to overdrive, the 4.88 would be perfect. 4.56 is good, and I am not changing them, I would efinately do 4.56 with 33s.

 

[dmoses42]

And just an idea, but if you are wanting to drive it around as-is for a while, just don't use the overdrive. It's computer controlled, just turn off the overdrive and you are basically running a C4, 1 to 1 ratio in 3rd. I may be wrong, and if so, someone will correct me.

As a side note, I'm going to run 32s with an AX15 so I was torn between 4.11, 4.30, and 4.56 gears. I bought 4.56 for 2 reasons, 1) I may want to go up to 33s, and 2) when I bought my gears 4.56 gears were significantly less than 4.30/4.27 gears. And I bought Motive gears based on positive comments I saw on here. I can't comment on them yet, they are still in the box right now....

 

[flousberg]

Mountains I just flip overdrive switch off. It is way better than a c4, no doubt. Had it for about ten years love it.

 

[davisjstone]

And just an idea, but if you are wanting to drive it around as-is for a while, just don't use the overdrive. It's computer controlled, just turn off the overdrive and you are basically running a C4, 1 to 1 ratio in 3rd. I may be wrong, and if so, someone will correct me.

As a side note, I'm going to run 32s with an AX15 so I was torn between 4.11, 4.30, and 4.56 gears. I bought 4.56 for 2 reasons, 1) I may want to go up to 33s, and 2) when I bought my gears 4.56 gears were significantly less than 4.30/4.27 gears. And I bought Motive gears based on positive comments I saw on here. I can't comment on them yet, they are still in the box right now....

And just an idea, but if you are wanting to drive it around as-is for a while, just don't use the overdrive. It's computer controlled, just turn off the overdrive and you are basically running a C4, 1 to 1 ratio in 3rd. I may be wrong, and if so, someone will correct me.

As a side note, I'm going to run 32s with an AX15 so I was torn between 4.11, 4.30, and 4.56 gears. I bought 4.56 for 2 reasons, 1) I may want to go up to 33s, and 2) when I bought my gears 4.56 gears were significantly less than 4.30/4.27 gears. And I bought Motive gears based on positive comments I saw on here. I can't comment on them yet, they are still in the box right now....

Good call. I was thinking that’s the best way to get me on the road for now. Turn off the OD through my quick 4 to save the trans from continually hunting for OD.

My assumption is this will work fine as a temp solution before I change the gearing.

 

[Broncobowsher]

Good call. I was thinking that’s the best way to get me on the road for now. Turn off the OD through my quick 4 to save the trans from continually hunting for OD.

My assumption is this will work fine as a temp solution before I change the gearing.

Should be fine. You will just be underwhelmed with the performance of the new engine with the tall gearing. Make sure you have the convertor locking up in 3rd.