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So I bought this 98 gt 4.6l with some bolt ons on it. The guy said the kid before him had the rear end rebuilt. I’m buying a bama tuner and I need to know if he changed the gear ratio or not, I was in there but didn’t think to look at the print on the ring gear at the time but I did take a picture of the other side of it. Need some help finding my ratio thanks.

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hmmm . . .the most practical way I know to find out, involves raising the rear of the car, turning the driveshaft, and counting the rotations of the driveshaft and rear wheels, then doing the math
 

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How did you calculate it out? And what do you think the reason for my Speedo and tach are out of line for?
I used a gear rpm calculator. Plus, I have 3.31’s in my ‘95 and it runs around 2k rpm at 70 mph. I had 2.73’s and it under 2k rpm at 70 mph and I’ve also had 3.73’s and they cruised above 2k at 70. The speedo and tach work but aren’t accurate? If so, the needles have might have been removed and placed back on at one point. One time I was replacing a broken odometer gear so I had the cluster out and flipped on its face with the clear plastic cover off. When I pressed the new gear into place the needles got pressed against the the gauge face and wouldn’t move freely. So I removed them and pressed them back on the “pedestal” to where they would operate correctly. They worked but they were no longer accurate. I sent the cluster off to a guy and he recalibrated it for $50.
A gear change will mess with the speedo reading but it won’t have any affect on the tach needle. I just had a light bulb go off in my head. I believe if you decode the VIN you can see what the stock gear was.
If they were out of the differential you could count the number of teeth on the ring and divide that by the number of teeth on the pinion. Example: A 4.10 gear set has 41 teeth on the ring and 10 teeth on the pinion. 41 divided by 10 equals 4.10.

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Saw that somewhere was hoping I wouldn’t have to.
Pull out a floor jack, 2 pieces of tape for marking the wheel and shaft, and count the revolutions of the tire to the driveshaft like JBert said. It's quick, simple, and accurate.
90lxwhite is right on point. You should check yourself to make sure. 3.55s(if they were changed) will be really close to 3.27s as far as rpms go. So would 3.08s.
Also, like he suggested look on your door jamb tag. It will have an axle code there. That code will give you your factory ratio. If it was an original manual car it should be 3.08 and auto should be 3.27.
 

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Pull out a floor jack, 2 pieces of tape for marking the wheel and shaft, and count the revolutions of the tire to the driveshaft like JBert said. It's quick, simple, and accurate.
90lxwhite is right on point. You should check yourself to make sure. 3.55s(if they were changed) will be really close to 3.27s as far as rpms go. So would 3.08s.
Also, like he suggested look on your door jamb tag. It will have an axle code there. That code will give you your factory ratio. If it was an original manual car it should be 3.08 and auto should be 3.27.
I’m pretty certain that they came with 2.73’s as well because some report having them. The guy in post #10 on this link says the VIN code for 2.73’s and 3.08’s had the same designation in the VIN so that’s not too helpful.
 

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My car has 3:27 rear end. At 2000 rpms I’m at 60 mph. At 2400 rpms I’m at 70.
I just went for a drive and at 60 mph it is at 1,900 in 5th and at 70 mph it is 2,100. Mine has a T5 transmission, 3.31 rear gear, and a 25” tall tire. I had the factory tach needle calibrated not long ago so supposedly it’s fairly actuate. I think it might be “accurate plus or minus a few.” I’d have to look at the report again.
I bet this fella has 3.27’s.
When it had 2.73’s it would do 70 mph in 2nd.. With 3.73’s I had to shift to 3rd to do 60 mph. I probably still might have to with the 3.31’s but I haven’t tried.
Edit I just played with another calculator and it was right on the money.
 

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