'70 Skylark GS drive and driven gears

Discussion in 'The Bench' started by ddrieg, Mar 28, 2021.

  1. ddrieg

    ddrieg Member

    I'm rewriting this post because for some reason the site would not post even though I was logged in and it lost the post (with all of the detail info in it).

    I've scanned through all 10 pages in the forum after a search for the word 'speedometer', but didn't exactly find my answer. This is the short version of my question.

    What is correct for a '70 1GS455/GSX (with A/C) and TH400, 3.42 gears (assuming G60-15 with a 26.4 tire height)?

    a 36-39 housing, with a 15 and 39 drive/driven gear set
    a 40-45 housing, with a 15 and 40 drive/driven gear set


    a 40-45 housing with 17 and 44 drive/driven gear set
    a 40-45 housing with 17 and 45 drive/driven gear set

    Thanks in advance.
  2. john.schaefer77

    john.schaefer77 Well-Known Member

    For my car with 3.42's and G60 tires I used the 15 and 39 tooth combos. It is accurate. I will look in the assembly manual to see what the factory combo is.
  3. john.schaefer77

    john.schaefer77 Well-Known Member

    The assembly manual calls for 17 and 44. I ran it through the gear calculator and both combos are good. Think I went for the 15 drive gear because I couldn't find a 17. Hope this helps.
  4. ddrieg

    ddrieg Member

    Thanks John. Yes, it seems the 17 drive gear is harder to find. This will save me some $$$ since I can reuse my 36-39 housing. Thanks!
  5. ddrieg

    ddrieg Member

    Currently, I have 275/50R15s on the car which I understand are no longer made by BFG. :( These compute to requiring a 40.06 driven gear with a 15 driven.
    If I go with 235/60R15s, it would be a 39.6 driven gear with a 15 driven.
    In both cases, after rounding, I'd need a 15/40 which would require a new 40-45 housing (extra $).
    If I reuse my 36-39 housing, get a 15 and a 39, I've read that for every tooth on the driven gear, its about 3% or 2-2.5MPH difference.
    I can live with that. :)
  6. DasRottweiler

    DasRottweiler -BuickAddict-

    26" MT STREETS, 3:42 Rear gears, 15 Drive. 39 Driven. Got the 15 tooth TCI Drive gear at Summit Racing....
    If and when I step up to 3:73s, I'll need to change to a 42 Driven...Jim

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