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  •  » Gear change in rear end

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    10/15/2017 4:05 PM  #1

    Wag8142
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    From: Laurel Ms
    Registered: 9/01/2015
    Posts: 33

    Gear change in rear end

    what can I expect  going from 3:40 to 4:10 rear end gear ? I have a 302 with a AOD tranny .....

     

    10/15/2017 5:37 PM  #2

    Steve-G
    I FYI 24-7

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    From: Glen Rose,TX
    Registered: 4/01/2013
    Posts: 544

    Re: Gear change in rear end

    3.40 gears are a great street car gear. 4.10 gears with overdrive are very similar to the original c4 and rear end on the highway. Git up and go will be much improved.

     

    10/15/2017 6:57 PM  #3

    TKOPerformance
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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,679

    Re: Gear change in rear end

    You need to look at more than just final drive ratio in top gear.  First gear needs to be a consideration, lest it become too short.  With an AOD 1st is a 2.46 ratio, so you end up with a great 10:1 or thereabouts overall ratio in 1st gear.  In 4th gear those 4.10s will act like 2.73s.  Its a good combination. 
     

     

    10/15/2017 7:07 PM  #4

    Wag8142
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    From: Laurel Ms
    Registered: 9/01/2015
    Posts: 33

    Re: Gear change in rear end

    Thanks for the info . It is great  to have people willing to help someone such as myself .

    ​Thanks again 

       •Thread Starter
     

    10/15/2017 9:23 PM  #5

    6sally6
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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,932

    Re: Gear change in rear end

    What's your plans for the 3.40 gears?!
    6sallly6

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    10/16/2017 10:42 AM  #6

    Tubo
    2008 BASH V Host

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    From: Ft. Walton Beach, Florida
    Registered: 4/01/2013
    Posts: 1,841

    Re: Gear change in rear end

    Unless your doing a lot of track time, or only drive locally, you may end up being sorry for not keeping the 3.40's. As previously posted the 3.40 iz a great all round street gear.
    Just sayin, as it is your car and your choices.
    BTW Is that 302 carrying a bunch of mods, and is the rear 8", 9", or 8.8??

    Tubo

    If it ain't broke, I haven't modified it Yet
     

    10/17/2017 7:57 AM  #7

    TKOPerformance
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    From: Newark, DE
    Registered: 12/15/2014
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    Re: Gear change in rear end

    It really depends on what you want.  A 3.40 is a great all around street gear for a transmission without overdrive.  Its also a great street gear for a tranny with a 3.xx first gear.  The issue you have with a 3.40 and a trans with a 2.xx 1st gear and OD is that off the line performance is lackluster because your overall gear ratio is too low.  Then it lugs on the highway because you are too low in the torque curve at cruise. 

    Like I said, its all about the combination.  I have a 3.73 gear in my IROC with a T56 that has a 2.66:1 1st gear and a 0.50:1 6th gear.  It needs more rear gear.  1st gear doesn't pull as hard as it should, and at 80MPH in 6th it only turns 1,800RPM.  Great for mileage, but it means a downshift into 5th is an absolute necessity if I need to pass, or slow down with traffic and then need to accelerate back to speed again. 

    IMO a 4.10 is a better pairing with the AOD given the total picture. 

     

    10/17/2017 2:21 PM  #8

    whlnut
    I FYI 24-7

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    From: Brunswick, GA
    Registered: 4/02/2013
    Posts: 317

    Re: Gear change in rear end

    Tire size (diameter) also makes a difference when considering performance.

     

    10/18/2017 6:17 PM  #9

    TKOPerformance
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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,679

    Re: Gear change in rear end

    That's absolutely true.  A good thing to check out would be your cruise RPM with tire size, rear gear ratio, and OD ratio factored in. 

     

    10/18/2017 6:20 PM  #10

    Steve-G
    I FYI 24-7

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    From: Glen Rose,TX
    Registered: 4/01/2013
    Posts: 544

    Re: Gear change in rear end

    That's true.  Lots of on line calculator to do that.

     

    10/18/2017 8:14 PM  #11

    Steve69
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    From: Madison Wisconsin
    Registered: 4/01/2013
    Posts: 2,795

    Re: Gear change in rear end

    I had 4.11 and went to a 3.25 gear.   I have a t5.  A lot happier.

     

    10/19/2017 6:08 AM  #12

    Al Newman
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    Registered: 4/08/2013
    Posts: 222

    Re: Gear change in rear end

    Had 3:55 s behind a C4 once.  Lots of grunt but felt like it needed another gear.  Also would be in high gear before clearing an intersection from a normal driving takeoff.  The C4 would downshift to second upon flooring it but would not grab low gear.  Left a big dead spot.  Your AOD may be different, but you need to consider that you will experience similar dead spots.  That tranny design did not contemplate such a tallrear gear.
    Best
    Al

    Classic cars are full of surprises and almost none of them are good ones!
     

    10/19/2017 11:11 AM  #13

    TKOPerformance
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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,679

    Re: Gear change in rear end

    Steve69 wrote:

    I had 4.11 and went to a 3.25 gear.   I have a t5.  A lot happier.

    What T5 did you have though?  A Mustang V8 T5 is a terrible pairing with 4.11 gears, because first gear in that T5 is 3.35:1, so you end up with near a 14:1 overall 1st gear ratio.  I used to see guys with that setup at the track.  I knew what gears they had by where they had to shift into 2nd.  With 4.11s it was right about the 60' mark.  It killed the car's times.  With a stock ratio T5 a 3.72 is pushing it, and 3.55s are really about the best setup. 

    The Z Spec T5, however, was designed to allow the use of higher rear gear ratios by lowering first gear to 2.95.  I still think 4.11s are too tall for that setup though.  3.73s are about perfect.  That's what I'm running in my '89 GT project. 
     

     

    10/19/2017 11:12 AM  #14

    TKOPerformance
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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,679

    Re: Gear change in rear end

    Al Newman wrote:

    Had 3:55 s behind a C4 once. Lots of grunt but felt like it needed another gear. Also would be in high gear before clearing an intersection from a normal driving takeoff. The C4 would downshift to second upon flooring it but would not grab low gear. Left a big dead spot. Your AOD may be different, but you need to consider that you will experience similar dead spots. That tranny design did not contemplate such a tallrear gear.
    Best
    Al

    Yep.  Got that same combo.  Top gear has it taching way too high to cruise comfortably on the highway.  Around town and backroads its fine, but sucks down the gas, even with a 289. 
     

     

     

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