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C4 Transmission AOD - inquiry

miniwik

New Member
Joined
Oct 26, 2017
Messages
4
I have a 1976 Bronco with the C4 - Dana 20 - 4:10 gears - I need to get to less RPM at higher speeds - any suggestions ?
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
4.10s with an overdrive need a stock sized tire to work right in a Bronco. Think 235/75R15 in a modern size.

If you are running over a 31" tire, the overdrive is too tall even with the 4.11 gears.

Also skip the AOD, the 4R70W is the better choice.
 

garberz

Bronco Influencer
Joined
Jun 24, 2007
Messages
6,856
Loc.
Conejo Valley, Ca.
Why do you need less RPM? A little taller tire, drive a bit slower, cruising at 3000 RPM all day long. All of these are easy. I don't consider the AOD an upgrade over a C4, too much cost for an overdrive that you'll need to regear to use. I run a C4, 3000RPM gets me 63mph. For me, that's a comfortable cruising speed. And I drive mine a lot on the highway.

Mark
 

rguest3

Contributor
Bronco Guru
Joined
Dec 13, 2012
Messages
3,778
Miniwik - How often do you drive on the highway at 70+ speed?

What tire size do you have now?

Do you have a lift to fit a taller tire?
 
OP
OP
M

miniwik

New Member
Joined
Oct 26, 2017
Messages
4
Sourcing the 4R70W - Transmission - what year and vehicle matches to the Bronco?
 
OP
OP
M

miniwik

New Member
Joined
Oct 26, 2017
Messages
4
Not often but currently I have a 4" lift with 33' tires - at 50mph I am doing over 30000 rpms
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
I presume you mean 3,000 and not 30,000 RPM.
Something else is amiss here. The RPM isn't right for the tires and gears. Even with some converter slip. Because 4.11 gears and 33 inch tires are a great pair without overdrive. I ran that for years. Put in 4.88 gears with the overdrive.
 

englewoodcowboy

Lick Creek Restorations
Joined
Jul 25, 2010
Messages
4,200
You are going to get opinionated information all over the place on a topic like this and bottom line is you need to decide for yourself. Running 33's stock C4 and 4.11 gears at 70 mph you are turning 3095 RPMs. With an AOD and 4.11 you would be turning 1963 RPM at 70 and with a 4R70W you would be turning 2051 RPMs. Ideally when we build we like to shoot for 2100 RPM at 70 MPH. This puts most motors in the middle of their torque curve so when cruising and you come up on a slight hill or overpass, the vehicle will not be hunting gears. Now my opinion, if you are looking to spend wisely, I would opt for a 4R70W and leave your gears alone for now. Depending on your motors health and power level it may become a bit sluggish but you can correct that later on down the road (assuming you are still running a stock bronco 302). I would search for a 96 to 2000 Ford Explorer or Mercury Mountaineer 5.0 and buy the vehicle. Pull the engine and transmission, then you would have an excellent candidate for a motor build and get your 4R70W. To keep costs down, if the motor is decent health wise, do a compression test etc. and you can install the explorer motor, its efi system etc. and it will control the transmission etc. It is a popular swap on here.
 

Nothing Special

Sr. Member
Joined
Nov 25, 2016
Messages
804
Here's another of those opinions. I have a manual trans with overdrive, 33" tires and 4.10 gears. I think that's a pretty good compromise. Most of the time I think I'd actually prefer it with 3.55 gearing (I'm in OD by about 35 mph the way I drive), but when towing a trailer I wouldn't want to make my first gear any taller.

All of that said, that is with a manual. It's easy for me to keep the trans from hunting between gears. If I want it loafing along without much power readily available I can stay in OD. If I want more power on tap I can downshift. I'm not sure how it would all work out with an auto, but I suspect I'd still prefer 4.10 gears to 4.88s, but I probably wouldn't be happy with 3.55s and an auto.
 
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