i just want to make sure i get this right...i opened up my rear today to find out i have non posi 3.89's so to get the correct speedometer gears i need a 7 tooth drive and a 21 tooth driven right or do i need to bump up to a 22tooth driven??

you have a 389? never heard of it but the chevy dealer can usually tell you by the color of the drive gear and rear ratio

the rear axle is not stock...guy i bought it had to replace it cuz guy he bought it from crashed it...he told me it had 3.53's in it and on the ring and pinions and it has 3.53 itched into it but on the on the casing the gm stamped steel code gm5 is on there and according to the code that is a 3.89....tryed to do the math out of the car by spinning tires and all but just wasnt adding up and i figured fluid needed to be changed and was best way to find out so i opened it up but now i need to figure out the correct speedo gears...from what i can tell has to be a 21 tooth same as 3.73 but will be slightly off if i go a 22tooth be to much...

sounds like you know more about it than me how did you come up with these numbers ? iv all ways wanted to know how to calculate that.

few different ways i dont feel like typing it but here the links to some decent sources...

http://www.vibratesoftware.com/html_...rive_axles.htm

http://www.thirdgen.org/techboard/su...ifying-gm.html (Need Help Identifying GM 10 Bolt Rear End!)

thanks for responding but i was actually talking about the numbers of teeth on drive and driven gear iv heard there's a way to figure out how many teeth you need yourself but don't know anybody that knows how i just put the t56 in my car so as soon as i get the speedo problem taken care of i will need to know that

3.89 is a Ford ratio.... do you have a 9" in that car? Thats the only way you could have that ratio that I know of. A 10-bolt would be either 3.73 or 4.10 if its in the same 'range' as the 3.89... theres no 10-bolt ratio in between 3.73 and 4.10.

As for the speedo gears, if I remember right, you can go to the main 'Thirdgen Org' homepage, and look at the tech articles - there should be something in there to help you determine what speedo gears you need for a given rear end ratio. I can't remember the name of it, but its there, I've looked at it before.

actually gm did carry a 3.89 its rare...though i was tired last night when i counted those recounted today with a much brighter flash light i have 4.11's makes a heck of alot more sense...lol i know what speedo gears i need now...its funny guy i bought car from said they were 3.53 i was like what???? i looked up part number in center console and it said 3.73 but i knew rear end was change...then i open it up and the stamped code on the gears was gm5 look above at link i posted and it says 3.89's i found that exact info on the link about all over the place...anyways so needless to say i was a little confused that why i opened it up to count gears but when u havnt slept in 40 plus hours and its 1130 at night you tend to miss a few key details like a few gears lol...my bad...

This company is one of our competitors but this page from there site is very helpfull for that information.

http://www.partshp.com/speedometer_gears.htm

7/22 according to the sticky at the top of the page.

JamesC