How Do I tell what rear end gears I have?
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Old May 16th, 2010, 08:17 PM   #1
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How Do I tell what rear end gears I have?


Ok, so before you tell me that I am an idiot and to check the tag on the pumpkin, I got the car modified, the DEALER I bought it from had no idea if the rear end gears had been replaced. So, can I use tire height, rpm and speed? I know currently Im running between 2000 and 2200 RPM at 60MPH in 5th gear, but not sure of how to do the math here. Any suggestions ?


Thanks in advance!
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Old May 16th, 2010, 08:54 PM   #2
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if your running 2200 rpms in 5th gear at 60mph i would say either 373 or 410 gears i have 373 gears and i do 2000 rpms in 5th doing 60mph
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Old May 16th, 2010, 09:10 PM   #3
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I'm not sure of the details, but it should be possible to jack up the back of the car, spin the rear wheels, and then compare the number of wheel rotations to the number of driveshaft rotations, then do some match to come up with a ratio.
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Old May 16th, 2010, 09:27 PM   #4
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Originally Posted by shanec View Post
I'm not sure of the details, but it should be possible to jack up the back of the car, spin the rear wheels, and then compare the number of wheel rotations to the number of driveshaft rotations, then do some match to come up with a ratio.
Yep. If you have to turn the driveshaft 3 3/4 times for 1 tire rotation you have 3.73's.. If you have to rotate it just over 4x you have 4.10's
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Old May 16th, 2010, 09:29 PM   #5
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Originally Posted by shanec View Post
I'm not sure of the details, but it should be possible to jack up the back of the car, spin the rear wheels, and then compare the number of wheel rotations to the number of driveshaft rotations, then do some match to come up with a ratio.
yep. jack up the car, put a reference mark on the driveshaft, and have a friend spin the tires ONE COMPLETE ROTATION (no more and no less). the number of times your driveshaft spins is your gear ratio. of course, it's gonna be a lil difficult to see the difference between 3.73s and 3.55s. more than one reference mark will help with this.
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Old May 16th, 2010, 09:38 PM   #6
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That's how I was taught to do it too. If you have 3.73s, the driveshaft will turn three complete revolutions and 3/4 of another revolution. If you have 3.55s, the driveshaft will turn three full revolutions and slightly more than 1/2 of a revolution.

The easiest way to make sure you turn the wheel exactly one revolution is to put the valve stem at the 12:00 position, then mark the driveshaft and a point to reference it to on the diff case. Then, when the valve stem goes around to the 12:00 position, you're done.

One other thing: IF you don't have posi, both wheels will have to be turned by two people at the exact same speed. Otherwise your results will be false.
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Old May 16th, 2010, 10:12 PM   #7
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Originally Posted by carl418 View Post
That's how I was taught to do it too. If you have 3.73s, the driveshaft will turn three complete revolutions and 3/4 of another revolution. If you have 3.55s, the driveshaft will turn three full revolutions and slightly more than 1/2 of a revolution.

The easiest way to make sure you turn the wheel exactly one revolution is to put the valve stem at the 12:00 position, then mark the driveshaft and a point to reference it to on the diff case. Then, when the valve stem goes around to the 12:00 position, you're done.

One other thing: IF you don't have posi, both wheels will have to be turned by two people at the exact same speed. Otherwise your results will be false.
or you could just turn the driveshaft
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Old May 16th, 2010, 10:14 PM   #8
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Dont forget they make 3.90's, but slim chance its that
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Old May 16th, 2010, 10:39 PM   #9
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Originally Posted by 97blkgt View Post
or you could just turn the driveshaft
let me know how that works out for you
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Old May 16th, 2010, 10:53 PM   #10
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Originally Posted by Memphis View Post
Ok, so before you tell me that I am an idiot and to check the tag on the pumpkin, I got the car modified, the DEALER I bought it from had no idea if the rear end gears had been replaced. So, can I use tire height, rpm and speed? I know currently Im running between 2000 and 2200 RPM at 60MPH in 5th gear, but not sure of how to do the math here. Any suggestions ?


Thanks in advance!
Well if you don't want to jack the car up. Rpm = (mph x final gear ratio x 336*) / tire diameter.

You can figure out the rest from there.
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Old May 18th, 2010, 07:39 PM   #11
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Was wondering the same thing bought mine tuned. Today I went 60mph and looked at rpm's was 2000-2200.
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Old May 18th, 2010, 09:09 PM   #12
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probably 4.10's...

thats what i have and at 2000 rpm i am at 57-58 mph (in 5th gear)
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Old May 18th, 2010, 09:55 PM   #13
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Originally Posted by bone_crusher View Post
probably 4.10's...

thats what i have and at 2000 rpm i am at 57-58 mph (in 5th gear)
That must be nice. My auto rev's 2k at 58-59 in overdrive with the 3.31's.
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Old May 18th, 2010, 10:15 PM   #14
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Don't know about you guys but I'm running about 2800@65 with 4:10s.
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Old May 18th, 2010, 10:24 PM   #15
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Originally Posted by bamagt View Post
Don't know about you guys but I'm running about 2800@65 with 4:10s.
That sounds more accurate. Depending on tire size, you should be pushing 3k at 70 with a 4.10.
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Old May 19th, 2010, 04:34 PM   #16
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I just checked and I'm at 2400-2500rpms@ 65mph
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Old May 19th, 2010, 05:04 PM   #17
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go hold 2k in 5th gear or 3k in 4th and give me your speed then, tell me if you have a manual or auto and give me you tire size i can tell you what gears you have.
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Old May 19th, 2010, 06:06 PM   #18
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I did 60mph in 5th yesterday and it was around 2000-2200rpm it has the BF 18's and its an auto
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Old May 19th, 2010, 06:44 PM   #19
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Originally Posted by J07StangGT View Post
I did 60mph in 5th yesterday and it was around 2000-2200rpm it has the BF 18's and its an auto
You need the whole tire spec to calculate. That being said, you should have 3.73's based on that number. Your RPM's don't vary 200 RPM either if your tc is locked in. Are you at 2k or 2.2k?

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Old May 19th, 2010, 09:43 PM   #20
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Originally Posted by Memphis View Post
Ok, so before you tell me that I am an idiot and to check the tag on the pumpkin, I got the car modified, the DEALER I bought it from had no idea if the rear end gears had been replaced. So, can I use tire height, rpm and speed? I know currently Im running between 2000 and 2200 RPM at 60MPH in 5th gear, but not sure of how to do the math here. Any suggestions ?


Thanks in advance!
Stock gears in the 2005 manual shift are 3.55's. If the gears were changed, chances are you are running an aftermarket tune. Did you get a handheld with the car? Is your speedometer off? Anyhow, I've got 4.10's and my rpms are a bit higher.
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