Best ratio for my wants?

Discussion in 'Got gears?' started by BigSmo54, Feb 3, 2018.

  1. BigSmo54

    BigSmo54 Member

    right now i have original 10-bolt with a 3.08 auburn minispool. Id like to turn my car into 500 maybe 600 hp car one day.... i plan to run it at the strip every once in awhile but for the most part keep it for pleasure and shows... i have 26.6 in wheels on the rear. My first question is should i upgrade to a 12 bolt. (Guessing yes)... and then second what ratio? Ive been told 3.73 but would lower such as 3.42 or 3.55 suit my needs? Current has a th350 in it... but i plan to put in a 4 speed. I know there is a formula to figure out everything but would liek the opinions of you seasoned vets! Thanks in advance!
  2. BrianTrick

    BrianTrick Brian Trick

    If you have a 71-72,with the 8.5” 10-bolt,then you already have the rear that you need. Your ratios are 3:08,3:23,3:42,3:55,3:73,3:90,4:10,etc. if you are planning on swapping to a 4-speed,you need to determine which one you will be using. You would want a different rearend ratio with a wide ratio,like an M20,compared to a close ratio,like an M21 or M22. Are you running AC?
  3. Dwayne B

    Dwayne B Well-Known Member

    I have a 72 skylark it had 2.56 gear.I put in Eaton posi 3.42 gears.The car has a th350 .I really like the 3.42 I all most went with 3.73 .I have had my car on interstate and 3.42 is all I would want .
    BigSmo54 likes this.
  4. BigSmo54

    BigSmo54 Member

    No AC
  5. Dwayne B

    Dwayne B Well-Known Member

    I have 2 cars the one I put 3.42 in is A 72 skylark a.c. car with 8.5.It had 2.56 gears and open rear.I got A original sales book that shows all the rear axle ratios. 15177078123151175772811.jpg
  6. ap1672

    ap1672 Silver Level contributor

    Do you have a 70 8.2 rearend in your car?
  7. Dwayne B

    Dwayne B Well-Known Member

    It's got the original 72 8.5 in it.
  8. BrianTrick

    BrianTrick Brian Trick

    350 or 455?
  9. Dwayne B

    Dwayne B Well-Known Member

    My car has a 350 2 barrel .I'm in the proces of a 4 barrel and cam swap. The car in my profile picture is my other car it is a 72 350 Gs .I have had my gs for 19 years.
  10. BigSmo54

    BigSmo54 Member

  11. BigSmo54

    BigSmo54 Member

  12. BigSmo54

    BigSmo54 Member

    It looks like a 3.42 is gonna be my best option. But i guess ive got a whole other ball game to thinm about now. What tranny i want to put in haha... but still wondering if i shoukd upgrade my 8.2 to a 8.5?
  13. gusszgs

    gusszgs Well-Known Member

    If your going to be putting some serious power to it down the road, say 500+ HP and a sticky tire, then I would absolutely go to a 71-72 8.5 corporate 10 bolt. A TH350 will get the job done if built well.
  14. Dwayne B

    Dwayne B Well-Known Member

    BigSmo54 this is Dwayne .
    Hey I'm sorry for answering your questions.I didn't realize at the time to late at night. The 8.5 10 boilt is very tuff.
  15. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

    Here you go, play with this for awhile.

    Without OD, it's a tough call. Personally, if I were going to be driving the car any significant amount of time on the highway or interstate, I'd stick with the 3.08 gears. Conservatively, how much time do you plan to spend at the track as opposed to driving it on the highway? I've had 4.10s in mine for years but I also have a 200-4R trans. The best of both worlds and a surefire way to wake up a 350 not to mention highway cruising rpm around 2,500 at 70mph. My setup would not be necessary with a 455 built the way you plan to have one. It will have more than enough torque to overcome even the 3.08s. Now, if you are considering a trans swap in the future then seriously consider a 200-4R, then choose the rear gears accordingly.
    Harlockssx and BigSmo54 like this.
  16. pbr400

    pbr400 68GS400

    My 455 powered ‘68 is mild, gets to the track once or twice a year, goes high 13s when it dies. It gets driven on the street a lot. I have 2.93s, which are original, and that’s a great gear for the highway and all around use. It’ll light up the street tires at will. 3.23 or 3.42 is about as steep as I’d want without overdrive. Also, you are in charge of whether or not you break your rear end-until you apply wot and stick it down, you’ll never break what’s in it now. (Unless you act like a hoonigan). Build what you have and see how you like it, use the calculators, and see how much more gear you could put up with on the day to day driving.
    Mark Demko and Harlockssx like this.
  17. BrianTrick

    BrianTrick Brian Trick

    With your Buick 8.2”,you only have two aftermarket/new ratios available: 3:42 and 3:64. Ir doesn’t look like you are concerned with originality,so I would get the 71-72 8.5” 10-bolt. Then just pick the ratio that is best for you.
  18. DBS

    DBS Well-Known Member

    71-72 8.5" & 3.42 or 3.55.
  19. BigSmo54

    BigSmo54 Member

    Im interested
  20. knucklebusted

    knucklebusted Well-Known Member

    I'd go with an 8.5. If you car already has one, you are set. Then I would work backwards from there.

    1) What RPM do you want to motor to live at on the highway if you plan to drive it more than 5-10 miles on the open road?
    2) What transmission will you be running and what is top gear ratio?
    3) How big a rear tire will you be running?

    With that info, you can plug in all the numbers and come up with the exact rear gear that fits your needs.

    The formula for rear gears is as follows:

    (Rear Tire Diameter x RPM) / (336 x MPH x Final Drive of Trans) = Rear Gear Ratio. If you don't have an overdrive trans, you can toss out the Final Drive of Trans since it is 1

    Sample calc: (27.5 x 2500) / (336 x 70) = 2.92

    Another: (27.5 x 2100) / (336 x 70 x .66) = 3.72

    As for the posi unit, I put a TrueTrac in mine because it is smooth and requires no special additives.
    BigSmo54 likes this.

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