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Thread: Easy way to figure out differential gear ratio?

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    Veteran Wally's Avatar
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    Default Easy way to figure out differential gear ratio?


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    Hi guys,

    I'm not entirely certain what differential gear ratio I am running in my JDM 86 has a TRD 2way in it, though not sure what diff gear ratio thats in there.

    Does anyone know a way to work it out?

    On the highway it touches 4000rpm at 117km/h which is pretty high, I don't remember my ADM being like that.

    Any help would be great
    Cheers,
    Chris.
    1984 JDM Zenki Trueno GT-Apex Hatch
    4AGE 16V
    +Quads, HKS 264 cams, Microtech LT10s, TRD gearset gearbox, TRD2WAY LSD, fun!

    FOR SALE: JDM 4AG PACKAGE+ITEMS (ENGINE/GBOX/EXHAUST/ECU/SHIT)

    http://www.ae86drivingclub.com.au/fo...ad.php?p=15205

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    its a 4.3 wally. at about 3500-3700rpm, should be about 100km/h, and 4300-4500rpm should be about 120km/h.

    the kid had a similar thing as well, was unsure what his ratio was.

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    Veteran Wally's Avatar
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    Thanks mate.

    Yer 4000rpm and its dead smack on 117 (digital dash).

    Hoping the new TRD tranny gear set doesn't much around with it too much in top gear.
    1984 JDM Zenki Trueno GT-Apex Hatch
    4AGE 16V
    +Quads, HKS 264 cams, Microtech LT10s, TRD gearset gearbox, TRD2WAY LSD, fun!

    FOR SALE: JDM 4AG PACKAGE+ITEMS (ENGINE/GBOX/EXHAUST/ECU/SHIT)

    http://www.ae86drivingclub.com.au/fo...ad.php?p=15205

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    4000x60 = rev/h 240000
    240000/.861 (t50 5th gear ratio) =278745.6
    278745.6 = revs at diff' drive flange per hour

    roll out on 185 60 14 = 1.8146m
    1000m/1.8146 = 551.1 revs per klm.
    so 64478.7 revs per 117k

    278745.6/64478.7= x

    = 4.3xxx

    its nice to brush up on some infrequently used maths skills every now and them
    Last edited by slydar; 22nd December 2008 at 12:09 PM.

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    Senior Member Adsie's Avatar
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    Wouldn't your speedo be out once you change your diff ratio?

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    That is right. But the tacho is still true. So you can calculate your real speed knowing your revs, gearbox ratio, diff ratio, and tyre circumferance.

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    dodge way is to mark your tail shaft and tyre, turn your tail shaft one full rev and see how many the outside of your tyre does.
    wheelfriends.wordpress.com

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    Quote Originally Posted by Wally View Post
    Thanks mate.

    Yer 4000rpm and its dead smack on 117 (digital dash).

    Hoping the new TRD tranny gear set doesn't much around with it too much in top gear.
    from what i know, the TRD 3 set and TRD 5 set dont really change the final drive ratio in the gear box.
    i know for sure that the TRD 3 set doesnt touch the final gear.

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    Yer i think the 5th gear in TRD set is virtually if not the same.

    That is right. But the tacho is still true. So you can calculate your real speed knowing your revs, gearbox ratio, diff ratio, and tyre circumferance.
    mmm so would that mean it's displaying slower or faster?

    4000x60 = rev/h 240000
    240000/.861 (t50 5th gear ratio) =278745.6
    278745.6 = revs at diff' drive flange per hour

    roll out on 185 60 14 = 1.8146m
    1000m/1.8146 = 551.1 revs per klm.
    so 64478.7 revs per 117k

    278745.6/64478.7= x

    = 4.3xxx

    its nice to brush up on some infrequently used maths skills every now and them
    oh awesome.

    go a spreadsheet?
    1984 JDM Zenki Trueno GT-Apex Hatch
    4AGE 16V
    +Quads, HKS 264 cams, Microtech LT10s, TRD gearset gearbox, TRD2WAY LSD, fun!

    FOR SALE: JDM 4AG PACKAGE+ITEMS (ENGINE/GBOX/EXHAUST/ECU/SHIT)

    http://www.ae86drivingclub.com.au/fo...ad.php?p=15205

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    i can do simple maths, but not spread sheets

    anyway, your cars jdm? and the box is too? your speedo will be correct then, j' cars come with a 4.3.

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