Easy way to check rear ratio?

Discussion in '1974 - 1978 Mustang II Talk & Tech' started by CanadianCobra, Dec 9, 2003.


  1. CanadianCobra

    CanadianCobra New Member

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    Is there any way of checking the ratio of an 8" without ripping the pumpkin out? We don't really have a diff cover to take off and look into, so is there another simpler way? (all the jacks and stands are being used at this present time)
    The tag is saying 2.79s. But my calculations are saying 3.37 (if adjusted for speedo and tach being off then the closest would be 3.40) so I want to know for sure. Why would someone put the tag back on if they swapped gears though :bang:
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  2. Turbo II

    Turbo II Founding Member

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    A rough way is counting the number of driveshaft rotations for every rotation of rear wheel but it's sometimes tough to distinguish a 2.79 from a 3:00. Best way is to mark both with chalk to keep track. Only exact way I know is to count teeth on the ring and pinion :shrug:
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  3. CanadianCobra

    CanadianCobra New Member

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    Thanks, I thought there was something a ratio with the tiring spinning but I could quite remember. I'll probably rip the pumpkin out when I can though.
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  4. 77sleeper

    77sleeper GO BUCS! Founding Member

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    get your tire height here then go here and assume a 3.0 gear ratio as a starting point and adjust the the ratio to match your rpm
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  5. CanadianCobra

    CanadianCobra New Member

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    Thanks, yeah I used some other site to figure out that I have roughly 3.40s before... and which the site you sent me to told me. I never paid much attention to it before even though I knew that I was revving high for having "2.79s". I'll probaby check the pumpkin anyway to be dead certain... I'd like to know exactly so I can figure out how much the RPMs are going go up with I swap to some different gears.

    I'm just still not getting why somebody would put the 2.79 tag back on if they swapped gears.
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  6. COBRA 7

    COBRA 7 Founding Member

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    High reving may be caused by a small torque converter. The one in my car is like 10-11". Most full size cars in the 70's have a 13-14" converter.

    This is how you can tell your gear ratio.

    1. Jack up only one rear wheel. Take the transmission out of gear and rotate the tire until the valve stem is on the bottom.

    2. Put a chalk mark on the bottom of the driveshaft.

    3. Rotate the tire around two complete revolutions while watching the chalk mark.

    4. If the chalk mark goes around two times and about three quarters more, the the gear ratio is 2.79. If it goes around three times then the ratio is 3.00. If three and a half times then 3.50 . :flag:
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  7. JonDawg

    JonDawg Mstng2 supplied me with my pair Founding Member

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    wait, Why 2 complete times, instead of just one?
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  8. Johan

    Johan Member

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    With one wheel on the ground and not spinning, the ring gear turns half as much. 2 turns of the airborn wheel = 1 turn of the ring gear.

    The one wheel up, one down idea is meant for an open diff. If you have a good limited slip, you want to get both wheels off the ground and spin the wheel around 1 complete revolution. Both wheels will spin together. If you try this with both wheels off the ground with an open diff, when you rotate one wheel, the other wheel may rotate a little or not at all.

    Excuse me while I nerd-out a moment: As long as you keep track of how much everything's turning, you can still figure the ratio. The ratio is how much the driveshaft rotates divided by how much the ring gear rotates. The amount of ring gear rotation is the average of the two tire rotations. So if you add up how much the two tire rotate and divide by 2, you get the ring gear rotation. Divide the driveshaft rotations (assuming you've counted them all) by the ring gear rotations and you've got your ratio. Rotating one wheel twice or both wheels once just makes the math a whole lot easier. Forgive me.

    By the way, when counting the driveshaft rotations, you only have to be accurate to within about a quarter turn to know which ratio you have. (3-1/2 turns = 3.55 for an 8")
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  9. zwhitr

    zwhitr Founding Member

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    As far as rpms go if you have an auto you will experience some slippage. something to consider for high r's.

    I had a 2.79

    1st gear was 10 mph per 1000 (I remember when dusting my boss's 94 shinoda 6k in first gear :bs: (acutually he beat me) :bang:



    crusing 2200 rpm @ 55 give you any help there

    now I have a 3.4 trac-loc---- he sold his so no re-match
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  10. dastang2

    dastang2 Active Member

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    i was told by a ford tech that there is a rare rear end in some mustang 2's that is worth a lot of money. i think he said it was a positrac
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  11. Turbo II

    Turbo II Founding Member

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    Yeah, they sure seem rare enough. I figured up here in "the land of ice and snow" :canflag: they would be common. Three years looking in the yards and not a one :(

    BTW: The Ford term for them was "trac-lok" or "traction lock"... "posi" is a GM term (just some useless trivia)
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  12. CanadianCobra

    CanadianCobra New Member

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    Since I rememeber revving at about 3000RPM at 60MPH, I don't think I have 2.79s in the back.
    The whole thing that started this was that I wanted to see how much RPM change I'll have when I swap gears. I'm thinking some 3.80s with Auburn Gear. That way when I go to 15" tires it'll drop down to about 3.70 or so.
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  13. zwhitr

    zwhitr Founding Member

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    If I remember correctly 3000 was about 70mph now with the 3.40

    3800 pushing 4k is around 70 still gotta change the speedo gear to be sure

    but one thing is for sure ---3.80's will kill you on the highway must have OD


    z
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  14. CanadianCobra

    CanadianCobra New Member

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    Thats what my Blazer is for, that thing makes the perfect road-tripper. I can actually fit people in it and the thing gets almost double the gas-milage on the highway then it does in the city. :nice:

    And if I have to move the Mustang somewhere I supposed I could always swap the 3.00 gears I have lying in the garage.

    Thanks for all the replies you guys and gals.
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