Aod, Which Diff Ratio

[saford]

Hi all.

New to this site so hope i'm posting in the right place? I have a question i hope someone can help me with.
Its not Mustang related but it is Ford related. I'm mailing from down in Cape Town
South Africa where info is hard to come by locally, so are classic Mustangs unfortunately!!
Ive just completed a build of a 32 Roadster. I have fitted an AOD to a 302. I see this is a
common swop for the older Mustangs. I have a Jag rear end and also 20 inch rears with
295x40x20 's. My problem is the AOD changes up too soon and the car just feels like its
geared all wrong. I have set the pressure to 35psi in neutral and the box is working good. I think my problem is the rear diff ratio, its a 3,07. So my question is.. What would be a good rear end ratio to fit? The car is
used for around town mostly, not too much on the highway. Would enjoy more power off the line but
don't want it to rev its guts out if i do go on the highway. I was thinking of fitting a 3.54 or 3.77 ratio but not
sure if i should even go to a 4,09 with those 20's fitted? Parts down here in SA are crazy expensive so
don't want to get it wrong a second time! So if there's anyone that's done the AOD swop on their Mustang and is also running 20's on the rear any advice on a rear ratio would be much appreciated.

 

[The Shemdogg]

keep the ratio between 3.27 and 4.10

 

[Edbert]

Wheel size is less important to the math than tire size, I do not know the height of a 295X40X20, but I do know they are taller than my 245X45X17.

In any case, with an AOD and a tire size (height) of 25 inches here's a break down of estimated RPM at 60mph.

In 3rd gear
3.25:1 = 2,621
3.50:1= 2,822
3.89:1= 3,137
4.11:1= 3,314

In overdrive
3.25:1= 1,782
3.50:1= 1,919
3.89:1= 2,133
4.11:1= 2,254

When I chose a rear gear I used that info/math to try and have it near red-line in 3rd gear for a 1/4 mile run. I chose a 3.89:1 rear-gear.

 

[Moosee1955]

33 is the actual recommended line pressure on ford aod. But 35 is close enough. 33 might make it shift even earlier. Your tire size if taller than what Ford intended from factory, will definitely make the gearing feel too tall. I'm no automatic trans expert, but why would it affect shift points. You've got an overdrive trans, do unless you drive over 85 mph on highway, you can gear it down to 3.56 to 3.73 range, no problem. I had a 89 Lincoln mark 7 with an aod. I think mine was geared with a 3.33 or so. In od, it tached about 2200 rpm at 70 mph. My problem was stock my 5.0 didn't have the torque to pull it proper. You've got extra horsepower and way less weight than I did. You might want to look into why it's shifting early. Light throttle mine shifted in od at 41 mph.


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[Robert Campbell]

33 is the actual recommended line pressure on ford aod. But 35 is close enough. 33 might make it shift even earlier. Your tire size if taller than what Ford intended from factory, will definitely make the gearing feel too tall. I'm no automatic trans expert, but why would it affect shift points. You've got an overdrive trans, do unless you drive over 85 mph on highway, you can gear it down to 3.56 to 3.73 range, no problem. I had a 89 Lincoln mark 7 with an aod. I think mine was geared with a 3.33 or so. In od, it tached about 2200 rpm at 70 mph. My problem was stock my 5.0 didn't have the torque to pull it proper. You've got extra horsepower and way less weight than I did. You might want to look into why it's shifting early. Light throttle mine shifted in od at 41 mph.


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What is your tire diameter?