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Gear Ratio fo 6 Cylinder

Transmission and Drivetrain

  1. money pit

    money pit Well-Known Member

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    Does anyone know what the gear ratio is for a 7 1/4 rearend for a 6 cylinder Barracuda. I replaced the slant with a 360. I eventually want to go to an 8 3/4 rear end. I was just curious what's in the rear end now. Is there a tag on the diff or something on the broadcast sheet that would tell me the gear ratio? I have the sheet but it's it pretty rough condition. Thanks
     
  2. moparlee

    moparlee Well-Known Member

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    Jack up the rear. As you rotate one wheel one revolution, count how many times the drive shaft rotates. That will be your gear ratio. You will have to approximate the last partial rotation as it will be a fraction, 1/4, 1/2, 3/4 etc.
     
  3. slantfish

    slantfish Member

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    either the 2.76 or 3.23 gears, I haven't seen any other factory combo with the slant. personally I would go with 3.55 gears. be aware that the 7 1/4 was pretty weak and a 318 could easily break one, so putting a 360 in front is a good recipe to kill the chunk early and parts for the 7.25 aren't as easy to come by. I'm stashing away my 7.25, found a 489 case 8.75 for $300 and putting in the 3.55 set.
     
  4. /sixchall

    /sixchall Well-Known Member

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    My ‘70 Challenger Slant6 car when new had a 7 1/4 rear end with a 2.76 gear set one legger. Spider gears gave out and I was fortunate to find an 8 3/4 with 3.23 gear set one legger. The rear end was complete 11” drum to drum, 742 drop out for an E-body for $50.00. Works fine for me.
     
  5. volunteer

    volunteer Member

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    Your final-drive ultimately is determined by your tire size. My Signet came with 2.76 ratio and 13 inch tires. The f.d. worked out to be about 3.10 - - then I went to 14 inch ralleys and f.d. dropped to about 2.9 - - . I now have a 2.93 unit and with my 14 inch Toyos, ratio back up to the 3.1 range.
    Remember that IF your tire diameter is 26.8 (or very close) your final drive ratio will be exactly what your ring and pinion are fixed at. (Could be explained another way I suppose - but some of you will get the 'gist' of it).
    Much easier to measure the tire circumference and then divide this number by Pi - or approx. 3.14
    Sure-grip units are available for the 7.25 but still not viable for anything larger than stock 318. As far as ratios, suggest nothing higher (numerically) than 3.23. I had stock '74 Cuda 360 over 40 years ago, with 3.55 rear and it revved over 5500 at only 105 mph. Not fun on today's highways.
     
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  6. aussiemark

    aussiemark Well-Known Member

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    7 1/4 rear axles are great as long as the drive shaft is not attached and you are only using it to push the car around the workshop they are perfect for this but nothing else.
     
  7. money pit

    money pit Well-Known Member

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    Thanks everyone for the info.