ring gear ratio change

Discussion in 'Classic Mustang Specific Tech' started by TOM B, Dec 17, 2003.


  1. TOM B

    TOM B New Member

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    I have a 65 vert that has a 347 stroker with Edelbrock heads, Blown Paxton sn95 and a Holley commander 950 system be built. the rear is a 8 inch, I dont want to change but would like a better ratio, It now has a 2.80:1 I would like around 3.80:1
    Is anyone running something close to this rear ratio and how would the trany shift and what would be my rmps be at around 60 mph. I am also thinking of getting a 2500 stall converter too.
  2. 12sec67

    12sec67 Active Member

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    depends on tranny your running and the last gear ratio.

    www.smokemup.com has auto math where you can check your gear ratio with tire size and tranny that you want to use.

    if you need a login - pm me
  3. gp001

    gp001 Founding Member

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    Give me what trans you have and your rear tire size and I'll make you a chart like this one

    http://www.2mfffabshop.com/mycar.htm
  4. johnkn

    johnkn New Member

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    Go here

    http://davewin.com/tech/gear_ratios.shtml
  5. brianj5600

    brianj5600 Active Member

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    don't waste money on an 8 in, it won't last.
  6. John Z

    John Z Founding Member

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    Since you ask, I'll give you a little math. It is not exact because it sounds like you have a C4 automatic which does not lock up when cruising. Sorry if you are already aware of the following.
    Your rpm will change by the ratio of the ratios - 3.8/2.8 = 1.375 so your rpm's will increase by 37.5%. If you're pulling 2000 rpm at 60 now, with the 3.8 you'll turn 2000*1.375=2700 rpm. That is quite a jump and fuel mileage and engine wear will increase. Now if you put in an over drive transmission with about a 0.67 final drive gear, your rpm's will go 0.67*2700=1800 rpm and you will still have the acceleration advantage of the 3.8 gear!

    With the mods you have done to the engine, I'm suprised you want to stay with the 8" rear. Remember, Ford only used the 8" for up to about 220 hp with the C code engine, a car ordered with the K code, 271 hp, got the 9". So Ford apparently thought the 8" was only good for something less that 271 hp, and that was with skinny tires that did not have the grip of today's rubber. I know many feel the 8" is OK for up to 400 hp, but it shoulds like you have way more than that. I agree with brianj5600 - go to the 9".
  7. Fostang

    Fostang Founding Member

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    How do you like the holley comander system. I was going to get that or the F.A.S.T. but ended up with the Tweecer. I haven't checked pricing on the commander what are they running at?

    I'd stick with a 3.25 rear with the blower you'll get up too high and too quick in the rpms with anything more.

    But if your engine can scream till 7500 rpm then go for higher.:)

    I'm going with 3.55 with a wide ratio tranny because I know the engine can got 7k no problem and still make power with the 2.08 valves.
  8. brianj5600

    brianj5600 Active Member

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  9. TOM B

    TOM B New Member

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    SORRY about the typo it a sn 89. Im running a c4 and 235-60-14 futura tires.
    I plan on a 2500 stall converter. this should help you all to calculate. is there a posi made for the 8 inch, it original and I would like to keep something thats factory
  10. gp001

    gp001 Founding Member

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