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3.73 or 3.90?

This is a discussion on 3.73 or 3.90? within the Drivetrain forums, part of the LSx Technical Help Section category; I have a Stock A4 and i was looking at getting some diffrent gears but i dont know if i ...

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    3.73 or 3.90?

    I have a Stock A4 and i was looking at getting some diffrent gears but i dont know if i should go with the 3.73 or 3.90 gears? all i plan on having done is headers and ORY and a cam later. Its my DD so i dont wanna ruin my gas mileage and i still take it on the highway(not offen tho).
    This is the package i was planning on going with ether way,

    http://texas-speed.com/shop/item.asp?itemid=54

    I think that is everything needed for the job correct?
    Thanks

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    Senior Member tatertot91's Avatar
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    3.73s are generally what people put on A4s
    Thats what most recommend
    3.90s would better be suited on an M6 along with 4.10s

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    I have 3.90s on my A4, love 'em. I don't feel the highway speeds are too high RPM wise at all. It's a pretty minor difference from 3.73's. no prob for a daily driver with either of them IMO.

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    I went with 3.73's on the Moser rear that I am awaiting for our A4. That ratio was recommended by most of the members on here. You can plug your numbers into this calculator if you want to check RPM's with various ratios: http://www.reiderracing.com/rtech-engine-rpm.php

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    i see its kinda split between the two lol good infomation tho, Im new to this so this might kinda be a newbie question but how can i find what carrier series i have? would it be a Series 3 because its newer (99) and a ls1 or is that pointless?

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    Quote Originally Posted by empire_942 View Post
    i see its kinda split between the two lol good infomation tho, Im new to this so this might kinda be a newbie question but how can i find what carrier series i have? would it be a Series 3 because its newer (99) and a ls1 or is that pointless?
    From everything I have read, upgrading the stock 10 bolt is a waste of time. If the car is street driven only, and not abused, you might be ok. They do make ring gear spacers to allow gear swaps in the event your carrier flange is not thick enough for what you are attempting.

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    Moderator Firebirdjones's Avatar
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    Depends on your goals for the car. 3.73's won't get you close enough to redline at the finish for optimum 1/4 mile times on a naturally aspirated engine. A power adder would be a different story.

    If gas mileage is your concern I'd probably leave it alone. As Jeff mentioned as well,,,I don't even bother sinking money in a 10 bolt.

    If your car has factory 2.73's and you are just dead set on more gear (in my opinion) you would be better off just picking up a complete rearend from another Fbody with either 3.23's or 3.42's. Plenty of them out there.
    It would be cheaper and much easier to swap.

    You'll probably find 3.73's and 3.42's will perform about the same at the track anyway.

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    Ya from what iv read i agree with not sinking money into the 10bolt. but how can i tell if my car is 2.73 or 3.23? i know that 2.73 is series 2 and 3.23 is series 3 tho, do u have to count the gears or can i look it up with the VIN #?

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    Assuming nothing has been changed, your RPO codes are on your driver door. There is a thread or sticky on here somewhere that deciphers all the codes for you. Our car has the 3.23 ratio which is RPO code GU5 if I recall correctly.

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    I have also heard that the 3.90 gear is a special cut that tends to be noisier than that of the 3.73 or 4.10.

    Not sure how much truth there is to this.

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    Quote Originally Posted by pajeff02 View Post
    I went with 3.73's on the Moser rear that I am awaiting for our A4. That ratio was recommended by most of the members on here. You can plug your numbers into this calculator if you want to check RPM's with various ratios: http://www.reiderracing.com/rtech-engine-rpm.php

    That calculator does not take into account your .70 OD in 4th....

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    Quote Originally Posted by pajeff02 View Post
    Assuming nothing has been changed, your RPO codes are on your driver door. There is a thread or sticky on here somewhere that deciphers all the codes for you. Our car has the 3.23 ratio which is RPO code GU5 if I recall correctly.
    Thank you ill look into that

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    i have 3.73's in my a4, had 4.11s they were alittle tall for the street never tried the 3.90's. just my 2cents

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    Quote Originally Posted by SOMbitch View Post
    That calculator does not take into account your .70 OD in 4th....
    The AOD calculates at 0.68 something, so it is pretty close.

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    Quote Originally Posted by pajeff02 View Post
    The AOD calculates at 0.68 something, so it is pretty close.
    Ummmmm I realy don't think so... it has me at 3K at 70mph and I actually spin about 2100:screwy

    Use the second one on here where you can enter final drive ratio :http://www.ringpinion.com/Calc_RPM.aspx

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    Quote Originally Posted by pajeff02 View Post
    Assuming nothing has been changed, your RPO codes are on your driver door. There is a thread or sticky on here somewhere that deciphers all the codes for you. Our car has the 3.23 ratio which is RPO code GU5 if I recall correctly.
    i tried to find the thread for the VIN guide an had no luck do you happen to remember which one it is??

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    Quote Originally Posted by 2000dreambird View Post
    i tried to find the thread for the VIN guide an had no luck do you happen to remember which one it is??

    GU2=2.73
    GU5=3.23
    GU6=3.42

    IIRC correctly LT cars had a GU4 that was a 3.08.

    BTW the VIN won't tell you this info it is RPO codes you look under....
    Last edited by SOMbitch; 12-18-2009 at 10:36 AM.

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    Thanx

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    i had 4:10s in a 93 a4, loved em, now 4:56 on the other hand....

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