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4r70 and axle gearing question

buckenbronco

Full Member
Joined
Apr 19, 2011
Messages
153
Loc.
Hanson
If I have a Duff 3.5 suspension kit and a 2 inch body kit I should be able to run 37 inch tires I believe. I'm switching to a 4r70 transmission. What should the axle gear ratio be? 4.56,4.88? Mainly an on road bronco
 

rguest3

Contributor
Bronco Guru
Joined
Dec 13, 2012
Messages
3,778
5 series gears for sure.

4.88 would be recommended for 35s.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
5.13-5.38

5.13 if you have a 351 and plan on a lot of flat highway driving.
Anything else 5.38.
 

rcmbronc

Contributor
Bronco Guru
Joined
Dec 15, 2003
Messages
2,692
Loc.
Tomah WI
I have 36" tire with 5.13 gears, 351W and 4R70W. Just about right. 4.88 would not be enough in my opinion.
 

Apogee

Contributor
Bronco Guru
Joined
Nov 26, 2005
Messages
6,033
I would probably run 4.88's if you have a torquey 351W or stroker based motor, as that puts you at about 2200 RPM at 70 MPH in OD. There's not much difference between 4.88's and 5.13's...about 100 RPM or 5%, so those would work just as well IMO.

If you're rocking a 289/302, I would probably go 5.38's.

Once I get my rig back up and running, it will be 351W/NV4500/Atlas 3.8/4.88's/37" tires that measure about 36.5", so I should have more input to provide then as to whether I wish I'd gone deeper on the gears or not.
 

Boss Hugg

Contributor
Bronco Guru
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Jun 8, 2010
Messages
2,140
I'm currently going to 5.13 for 35's. Going to be a little much, but it'll also cut down on my crazy highway speeds. with stock gearing I can't use the OD very much.
 

sykanr0ng

Bronco Guru
Joined
Aug 11, 2014
Messages
5,363
The only transmission commonly used in Broncos with higher overdrive ratio than the 4R70W is the AOD.
Take the gears you are thinking of using and multiply by 0.70 to get the effective gear ratio in overdrive.

4.88 in the 4R70W overdrive would be like running 3.42 gears in 1 to 1
With 37 inch tires that won't be fun for you.

More gear or smaller tires.
 
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