1970 Z28 engine questions

Discussion in 'Original Drivetrain Topic' started by gpm6367, Jul 13, 2019.

  1. gpm6367

    gpm6367 Veteran Member Gold Member

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    More drive time today. AC evacuated so I will start pulling off system this week.

    My pseudo grandson has taken a shine to the Z.

    I know from convo with original owner car has 3:73 rear diff. 3k rpm at about 52 mph? Does that sound right?
     
  2. gpm6367

    gpm6367 Veteran Member Gold Member

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    84C1D47C-2A70-494E-A80B-35C64E916973.jpeg C96E90BB-DB42-4584-95FE-BB4A1B2174B4.jpeg 28640B97-2CE3-47AF-A563-0936B095E7CD.jpeg A45A63EA-E8C2-4D3E-8B42-8810A5B332B0.jpeg Forgot pics..
     
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  3. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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  4. COPO

    COPO Veteran Member Lifetime Gold Member

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    With 3.73 at 70 mph you s/b reving 3500. My tach is out too, I rev almost 4000 rpm @ 70. My gears are 3.73 cause I took the cover off and looked.
     
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  5. shanghai Kid

    shanghai Kid Veteran Member

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  6. gpm6367

    gpm6367 Veteran Member Gold Member

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    I’m going to mark driveshaft to verify but not likely my rear diff ratio ever changed. If COPO is at 4K for 70 mph my 3k at 52 mph doesn’t seem “way high”, and that’s assuming both my tach and speedo are accurate.
     
  7. COPO

    COPO Veteran Member Lifetime Gold Member

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    I have 235-60-15 tires all around. Maybe the F60's would be dead on. My speedo is dead on when I checked it with an app and stop watch.
     
  8. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Let us know.

    I would say its a very high likelihood the gear's been changed. That's not just a little bit off.

    I assumed 26.5 inch tires in the calculator. That's about average for these cars.

    To make it work with a 3:73 gear you would need a 21.73 inch diameter tire! On a 15 inch rim that's only about a 3 inch sidewall!

    COPO's 23560 is 26.1 in diameter. An F60-15 is 25.9
     
    Last edited: Aug 5, 2019
  9. COPO

    COPO Veteran Member Lifetime Gold Member

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    I went for a drive today and at 50 mph I'm almost at 3000 rpm and I have 3.73 gears and took pics of the numbers when I had the diff cover off a few yrs ago. In the past I thought I may have had 4.10's that experimental swapped in but they didn't.
     
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  10. gpm6367

    gpm6367 Veteran Member Gold Member

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    At 50 mph I am about 2800 so I fail to see much difference. Have to check my tire size...
     

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