Drivetrain AOD require new rear end gear ratio?

Discussion in 'Classic Mustang Specific Tech' started by delmo408, Mar 8, 2013.


  1. delmo408

    delmo408 New Member

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    Looks like my next project will be adding a aod for the obvious reasons. I've had one source tell me that I will need to/want to change the ratio of the rear end. They also suggested JUST changing the gears and leave the trans alone to lower the higher speed rpms. Don't see why I would have to change gears because of the od gear, and seems like just changing gears will accomplish one goal while making new problems.

    Any thoughts?
     
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  2. rbohm

    rbohm Founding Member

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    you dont need to change the rear gear when swapping to an AOD. understand however that if you have say a 2.80 rear gear, going to the AOD will make it seem like you are trying for a top speed run at the bonneville salt flats when you are running down the highway. the overall gear ratio will be on the order of 1.87 or so. so you might want to consider changing the rear gear to something like a 3.73. that will give you good dig off the line, and still puts you rolling down the highway with an overall gear ratio of something like a 2.47 or so.
     
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  3. Falcon79

    Falcon79 Active Member

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    I have 3.80 gears and an aod in my mustang, nice burnouts and gets quiet around 55mph. Best upgrade i ever did.
     
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  4. delmo408

    delmo408 New Member

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    I don't have a tach. Based on your experience, what is your best guess at what my rpms will be at 60 mph assuming everything about my drive train is stock (289 and 3-speed auto)?
     
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  5. horseballz

    horseballz Member

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  6. delmo408

    delmo408 New Member

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    Slick! Thanks everyone.
     
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  7. Hwyman

    Hwyman Member

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    I have an AOD behind a 302 in a 66 coupe. I changed to 3.80 gears. I'm running 225/60 14 tires. At 70 mph I am at 2500 rpm. It is the perfect rpm for me. I went on a 150 mile cruise the other day with a car club. Holding the speed down to 55-60, and no hard acceleration, I got 18 mpg. Your original question was do you have to change gears when you add an AOD. The answer is no, you don't have to. The car will run fine, but be a little low on rpm in OD. But, by putting in an AOD you can change gears and really help the acceleration.
     
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  8. delmo408

    delmo408 New Member

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    Thanks Hwyman. Real good info.
     
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