gear ratio

Discussion in 'Transmission & Driveline Topics' started by slayer74, Sep 3, 2014.

  1. slayer74

    slayer74 Veteran Member

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    i have a 76 with a 383,th350 and 3:90s in the rear at 65-70mph i'm at 3500rpm,thinking about switching to 3:42s for better higway cruising (hopefully) want to be able to keep up with traffic and not be at high rpms- any suggestions thanks
     
  2. andy736

    andy736 Veteran Member

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    I run 3.42 with a 26" tall tire roughly. I'm at around 3k at 65. I try to stay away from the freeway as much as possible. 3.42 is a good overall gear to run. A taller tire may help some but, not much.
     
  3. andy736

    andy736 Veteran Member

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    I've been considering a od trans swap, but I just can't justify it right now. My TH350 is built pretty good for what I want. If you do a lot of freeway driving OD is the only way to go.
     
  4. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Something in the range of 3.00 ~ 3.23 if you're going to stick with a 3 speed auto. That 383 will have plenty of torque for the gear. But the best of both worlds lays with an OD tranny.
     
  5. big gear head

    big gear head Veteran Member

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  6. slayer74

    slayer74 Veteran Member

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    thanks everyone,tranny swop is out of the question just dont have the $ right now,figured a gear swop would be cheaper alternative
     
  7. Zspoiler

    Zspoiler Veteran Member

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    Mine is a 1979 Z-28 of which came a 350 engine / turbo 350 trans and 3:42 gears. I know because I have the build sheet.And its also the same set up as my 1981 El Camino SS. When I got the car it had a 26 inch tire (235/60/15`s) and cruised at 60 mph at 2800 rpm. If you a taller tire (a 255 or 275 /15 inch tires) to see what you like. Its cheaper once you change to the gears you like.Because it cheaper to buy a set of tires ,and or borrow some from your friends to try out.
     
  8. JodysTrans

    JodysTrans Veteran Member

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    Here is my easy RPM & Gearing formula.......

    any RPM x new differential gear / existing differential gear = effective RPM

    Simplified with your post input, I randomly picked 3500 RPM for the formula.

    3500 x 3.42 / 3.90 = 3069 RPM
     
  9. Fbird

    Fbird Veteran Member Lifetime Gold Member

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    100% correct as you are merely adjusting the RPM by some % relative to the original ratio.
    however, to simplify it a tad

    rpm x (current gear ratio / proposed gear ratio) can give you a quick visual answer. example: from 3.42 to 3.73 is a 9% change so you would get a 9% change in rpm. 3.42 to 4.11 = 20% change...etc.. same "change" formula for tire changes.... its all % change.

    3.90 to 3.42 = 11.4% change so a little better than 10% so about 3100 rpm with NO other changes.
     
  10. JodysTrans

    JodysTrans Veteran Member

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    Do you realize you have the formula reversed
     

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