Super-low Gear Ratio in a 7.25” Rear End??

Transmission and Drivetrain Tech

  1. 69DartDave

    69DartDave Well-Known Member

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    Okay, this is weird. I finally drove my son’s project car (’69 Dart, slant 6, A904, 7.25 rear) on the freeway for the first time, and the engine RPMs were waaaay lower than what we’d expected.

    If I calculate the 2.76:1 axle ratio and 1.00:1 3rd gear ratio and 25.7” overall tire diameter (225/55/16), I should be getting 2500 RPM at 70mph. See my chart below.

    Then I drove it: At 70 mph in 3rd gear, the tach shows only 2000 RPM. Maybe slightly less.

    Dropping the stock A904 into 2nd gear at 70 mph, the tach showed only 2700 RPM. Almost what I was expecting to get in 3rd gear!

    What in the heck? Is there a rear axle ratio numerically lower than 2.76:1 for a 7.25” rear end? The differential cover has no identification tag on it.

    If I reverse-calculate the rear end gear ratio to match what the real-world RPM was, it’s something like a 2.10:1 ratio!

    And to make it weirder, the car accelerates pretty stoutly off the line in first gear, which I would not expect with such a "granny gear" (granted,this motor is built up pretty extensively from the stock configuration. Even at 90 mph this car has plenty of power reserves)
     

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  2. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    Lockup converter maybe?
     
  3. cdavis

    cdavis Well-Known Member

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    I had a 7-1/4 one time that had a 2.45 gear. I think it was out of a '75-'76 though.
     
  4. dartfever

    dartfever Well-Known Member

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    Is it the original trans? If it's an A998 or A999 the gear ratios are 2.74 first, 1.54 second and 1.00 third. Might explain the decent off the line performance a bit. And it would be a lockup TC as RRR mentioned.
     
  5. IfItsGotWheels

    IfItsGotWheels X S Tech

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    Defective tach??
     
  6. 69DartDave

    69DartDave Well-Known Member

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    I can vouch that the tachometer is perfectly good. In fact, it's part of the Holley Avenger EFI system that the car has, read off of the hand-held computer, so it's gotta be a very robust signal from which to meter the fuel injection (it runs great!). Plus, my ear is calibrated well enough - that motor is simply not revved high at 70 mph.

    It is the stock 1969 A904 transmission.

    And a locked-up torque converter would only make my chart more accurate. If it's not locked-up, then it'll slip somewhat, right? - making the engine RPM higher than otherwise.

    I'm certainly not complaining about this lower-than-expected cruise RPM. In fact, we were researching what it would take to retrofit a A998 4-speed Automatic into the car to lower the cruise RPM. This will be my son's daily driver, to include lots of freeway miles, and fuel economy is a factor. Looks like we don't even need a 4-speed!
     
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    • pishta

      pishta I know I'm right....

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      2.26:1 - Used in many civilian M-bodies over the years
      with a lock up you would see 2.26 on the dot with your math.

      Aftermarket gears available.
      2.26:1
      2.76:1
      2.94:1
      3.21:1
      3.23:1
      3.55:1
      3.90:1
      4.10:1
      4.11:1
       
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      • 2 Darts

        2 Darts A-body Addicted

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        There was both a 2.45:1 and a 2.21:1 7¼. The cases used for these are different from the others. Both used nine bolts to secure the cover. Most 7¼ rear covers were fairly round. The cover for the two lower numeric ratios was distinctly oval shaped.

        By my calculations, a 2.45:1, 255/55-16 would require 1919 rpm for 60 mph.
         
      • Pascamp

        Pascamp Well-Known Member

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        I think the Holley Avenger is a TBI designed with a V8 in mind. That might explain the discrepancy. Kinda like the old 4/6/8 tachs.
         
      • Leadfoot

        Leadfoot Jane Goodall of the N&P Forum

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        Did you ever lift the back end and count the turns?
         
      • SGBARRACUDA

        SGBARRACUDA ROY FABO Gold Member

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        Yes, I had a Mirada with a 7 1/4 and 2.20 gear.
         
      • 69DartDave

        69DartDave Well-Known Member

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        Thanks for the feedback!

        Interesting theory about the tach signal being applicable for an 8-cyl motor. The ratio of 6/8 is about the same as 2.10/2.76 ! Plausible theory that my tach is indeed “off” for this particular car. I should call the Holley tech support line and get their thoughts.

        Also, that’s worthy of note that such numerically-low gears were indeed available for this rear end. I’ll check the shape of the differential cover. Maybe it has 2.21:1 rear end!

        I’ll jack up the rear tires, spin the driveshaft exactly one complete revolution, and measure how many revolutions the tires spin. That should be pretty darn conclusive (assuming I make careful measurements).
         
      • Pascamp

        Pascamp Well-Known Member

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        If you look at the discrepancies in percentages I think that's what's going on here. The ECU is reading the input from a 6cyl but assuming it's an 8cyl.
         
      • Johnny71dusty

        Johnny71dusty Well-Known Member

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        Where did you see these ? I can’t find anyone selling gears for A body 7.25 in anything bud 4.11 . I’m trying to get in that 3.23-3.55 area and can’t find .
         
      • pishta

        pishta I know I'm right....

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      • Johnny71dusty

        Johnny71dusty Well-Known Member

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      • pishta

        pishta I know I'm right....

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        IIRC the ring and pinion were the same, the spiders and the carriers were different on the later Dakota 7.25 rears..but don't buy until someone with more 7.25 experience chimes in.
         
      • Johnny71dusty

        Johnny71dusty Well-Known Member

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        Summit says that r&p is for 80s cars and up . So I don’t know what to do lol . My 2.76 is really sad
         
      • Johnny71dusty

        Johnny71dusty Well-Known Member

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        And that first link you sent even says “ no longer made “ and they are not selling them .
         
      • 33IMP

        33IMP Well-Known Member

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        Has nobody pointed out that a 2,2x to 2.7x is a very HIGH gear, not a super low one? A super Low gear for a 7 1/4 would be a 4.88.
        Low gear=higher number (low vehicle speed for given rpm.
        High gear= lower number (higher speed for given rpm).

        Edit: uh, nevermind. Five year old thread.
         
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        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Well it's "too easy" to find out

          If it's not limited slip, jack up one rear wheel in neutral, turn the wheel TWO turns and count the driveshaft revolutions. You should be able to estimate 2.x turns

          Limited slip jack up both wheels, turn one of em 1 turn and count driveshaft
           
        • RustyRatRod

          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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          Yeah well......lol
           
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