muncie speedo bullet and adapter 1970 Z28

Discussion in 'Transmission & Driveline Topics' started by Jeffmort, Mar 22, 2017.

  1. Jeffmort

    Jeffmort Veteran Member Gold Member

    436
    10
    Jan 7, 2004
    Encinitas, CA USA
    Mark-
    That is the same adapter and gear color that is in my car. Only difference is that my car has, 26.6" height rear tires (245/60-15 which are 1" taller than stock tires (stock height is 25.6"; E70-15). My assumption is that the adapter solves for stock height rear tires - am I correct in thinking this?
    If I reference my handy speedo gear calculator page, it tells me that I need a black colored, 23 tooth driven gear with (8 tooth drive gear in transmission), 3.73's, and 26.6" tall tires. I assume that calculating it this way, I would NOT use the adapter with the "natural" colored gear which is designed for stock height tires and that I should eliminate the adapter, screw the speedo cable directly into the bullet, and use the black gear. Does this thinking make sense or should I use the adapter and natural gear even though my tires are taller than stock?
    Here is a copy of the speedo gear calculator:
    speedo gear.PNG
     
  2. 70lt1z28

    70lt1z28 Veteran Member Gold Member

    2,253
    320
    Oct 3, 1999
    Beavercreek, Ohio, USA
    Just a hint on this also. Measure the tire diameter from the center of the rear axle (hub) to the ground with the tires at the correct pressure. Than multiply this by 2 to get the true rolling diameter. It can be an inch or so different than if you measure left to right on the tire.
     
  3. COPO

    COPO Veteran Member Lifetime Gold Member

    16,521
    301
    Sep 15, 1999
    Ontario, Canada
    The gear is cheap to buy. Buy the black and give it a shot, it should work without the SW adapter.
     
  4. Jeffmort

    Jeffmort Veteran Member Gold Member

    436
    10
    Jan 7, 2004
    Encinitas, CA USA
    That's the conclusion i came to as well - good to get a confirm.
    OI am going with black gear without adapter and see where that takes me and will adjust from there.
     
  5. COPO

    COPO Veteran Member Lifetime Gold Member

    16,521
    301
    Sep 15, 1999
    Ontario, Canada
    Keep us posted.
     
  6. Jeffmort

    Jeffmort Veteran Member Gold Member

    436
    10
    Jan 7, 2004
    Encinitas, CA USA
    Here is an update for those who might be interested.
    I installed the "speedo bullet" without using the stock adapter and my speedo is reading correctly.
    I used the chart/calculator above to figure out which color/tooth count driven gear I should use and it worked - my speedometer reads accurately.
    So my conclusion is: if your car is 100% stock (stock height tires, stock gear ratio, stock speedometer, stock transmission) then use the stock adapter and the stock colored/tooth count driven gear and you should get an accurate speedometer reading.
    If any of these pieces of the puzzle (non-stock height tires, rear gear ratio, etc.) are not stock, then you can and probably should eliminate the adapter and go by the chart I posted above in order to get your speedometer to read accurately.
    Thanks again for all the input.
     
  7. COPO

    COPO Veteran Member Lifetime Gold Member

    16,521
    301
    Sep 15, 1999
    Ontario, Canada
    Thx for letting us know Jeff. Good 1.
     
  8. 1970_Z28_503

    1970_Z28_503 Member

    84
    5
    Apr 12, 2017
    Oregon
    My 70 z M22 didnt have adapter. Speedo was crazy off when i got car so I put in a 24 tooth yellow gear and its tits now. 26.6 tire dia.
     
  9. FlaJunkie

    FlaJunkie Veteran Member Lifetime Gold Member

    6,066
    113
    Mar 24, 2001
    Rockledge, Florida
    I put a spedo gear in it for the 3.73 axle and have checked it with GPS...right on. Mine didn't have an adapter on it but I bought it a year old. Could have been removed by the previous owner.
     
    1970_Z28_503 likes this.
  10. restore-z28

    restore-z28 Veteran Member

    361
    57
    Jul 4, 2010
    Vancouver, BC
    If anyone is wondering what one of these reducers looks like inside here you go. This one is the original from my '70 Z. Very easy to rebuild, took the 4 screws out, a couple of light taps on the side to break it loose, cleaned the old grease out (what was left), re-packed with fresh grease and reassembled. Good thing I took it apart as it needed grease, the gears were still good with nothing broken.

    IMG_20180301_094141.jpg IMG_20180301_094155.jpg IMG_20180301_094500.jpg
     

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