Formula for engine RPM for rear axle ratio.

Discussion in 'Transmission & Driveline Topics' started by john Gialanella, Nov 5, 2020.

  1. john Gialanella

    john Gialanella Veteran Member

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    Does anybody remember the formula for rear end axle ratio. My numbers matching 1970 SS 396 Camaro L-34 was given only one rear axle ratio 3:31 to 1. I have the original 14 x 7 rally wheels silver color, the tires are Firestone Firehawk Indy 500 225-70-14. The diameter is about the same as the F-70-14 Firestone Wide Ovals which is 26.3 inches. The Engine, TH 400 and rear axle are original, but I do not know if the gear ratio was changed. When I take it out for drive on the Highway at 80 MPH the Tachometer reads 3,750 RPM. That seems high for 3:31 to 1 gears. I have two floor jacks, but do not want to jack up rear and measure how many revs of drive shaft for one revolution of the tire. If I can get the formula I can see if it does have the original rear gears.
    Thanks John.
     
  2. PBZ28

    PBZ28 Veteran Member

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    Try wallaceracing.com. The calculator has worked good for me.
     
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  3. Jodi

    Jodi Veteran Member

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    (Gear ratio) 3.31 x 80mph x 1.1 (transmission final ratio) x 336 (given number) = 3721 RPM
    26.3 (tire dia.)
     
  4. john Gialanella

    john Gialanella Veteran Member

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    Jodi.
    So it seems that I have my original 3:31-1 rear gears.
    Thanks to PBZ28 and Jodi.
     
  5. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    Final drive ratio would be 1 and not 1.1 in the formula I should think. That change alone yields a calculated RPM of 3383.

    I think that either John’s speedo and or tach may be out of calibration or the rear ratio has been changed.
     
  6. Jodi

    Jodi Veteran Member

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    You're absolutely correct... going by memory, I got a little ahead of myself and started plugging in all the numbers from the OP post.


    Thought this reads 3:31 to 1. 1
     
  7. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    I literally laughed out loud at your post...not laughing at you but with you. Who hasn’t misread something and then proceeded down a path based upon that misread?

    so if my RPM calc is correct then John’s is reading 367 RPMs too high ...give or take. Or the Speedo is wrong or the gearing has been changed. Hard to say which exactly at this point - I tried subbing in known ratios but couldn’t get to 3750 RPM
     
  8. Jodi

    Jodi Veteran Member

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    ^^^LOL :D:D:D...
    3.73 yields the closest to 3750RPM, which is 3812 RPM.
    Perhaps the 3.31 was replaced with a 3.73, which is a very common swap since the same carrier can be use.
     
  9. flowjoe

    flowjoe Moderator Staff Member Lifetime Gold Member

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    Yeah, I thought about that too - since it was the closest in RPM. Really only one way to know for sure...:bowtie: (hint-hint John)
     
  10. COPO

    COPO Veteran Member Lifetime Gold Member

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    Take the cover off then change the fluid.
     

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