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Can a 95 9c1 rear end with a 3.73 gear differential fit on a 93 Buick road master with the tow package? Please help!!
 

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Yes it will work. You will have to re-calibrate your spedo, but I believe the TBI cars did this via a new gear in the tranny. No re-program necessary. 3.73:1 ratio should not cause you any troubles with vibration unless something is not correct to begin with. The disk will work fine. If you do the brake bolt mod it would make the discs more effective.

If you have the tow pack what rear do you have now? Some of the Roady Sedans (assuming we are talking about a sedan) tow pack cars came with 3.73:1 gears.
 

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Yes it will work. You will have to re-calibrate your spedo, but I believe the TBI cars did this via a new gear in the tranny. No re-program necessary. 3.73:1 ratio should not cause you any troubles with vibration unless something is not correct to begin with. The disk will work fine. If you do the brake bolt mod it would make the discs more effective.

If you have the tow pack what rear do you have now? Some of the Roady Sedans (assuming we are talking about a sedan) tow pack cars came with 3.73:1 gears.
Correct but it well documented that the stock driveshaft will cause vibration that the 3.08s don't really show until it spins faster with the 3.73's. The 3.73 will show a problem with the DS that the 3.08 never had.

I never really noticed a problem with vibration when cruising up to 80-90 MPH until I swapped in another rear with 3.73 gear. With 3.73 I would get a small but noticeable vibration at about 65-70 MPH. Switched over to a balanced Dynotech DS and the problem was instantly fixed.

BTW - check for posts by member Goldmaster. He has done this swap in a RM before and may have some info that would be helpful.
 

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Thank you both for that info, it is a 93 RMS and it has a hitch on the back so I'm assuming I have at least 2.92's in it now. I doubt that it had the 3.73's in it already because it doesn't pick up fast enough to have that feel to it and it currently has drum brakes on it


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How is the gas mileage on the 3.73's?


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It will go down a little, but there are obvious trade-offs as you will turn more RPM's when crusing. You are also making the car more spirited to drive and your right foot will help dictate how many MPG's you will lose.

This has been discussed quite a bit before. Just get familiar with the search function and you will find lots to read.
 

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If you do have a vibration problem which seems to have occurred in about half the cases IIRC, the first solution you should try is to disconnect, rotate the shaft 180 degrees, and reconnect. This works sometimes and may save you incurring the expense of rebalance/replacement.
 

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Can a 95 9c1 rear end with a 3.73 gear differential fit on a 93 Buick road master with the tow package? Please help!!
If you go here: http://www.transmissioncenter.net/, and find "speedometer world", click on it, and you will be able to get the proper gear ratio for the trans to correct your speedo for the gear ratio and tire size. Depending on what driven gear you need, I have a few for sale.
 

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If you do have a vibration problem which seems to have occurred in about half the cases IIRC, the first solution you should try is to disconnect, rotate the shaft 180 degrees, and reconnect. This works sometimes and may save you incurring the expense of rebalance/replacement.
I usually marked the DS so I installed it back as original. When I did the rear swap to 3.73 I did try to rotate the DS first but the vibration got worse. Flipped it back around to where it was livable until I could swap the DS and been vibration free since. It was actually a pleasure to drive on the TP doing 90+

If you go here: http://www.transmissioncenter.net/, and find "speedometer world", click on it, and you will be able to get the proper gear ratio for the trans to correct your speedo for the gear ratio and tire size. Depending on what driven gear you need, I have a few for sale.
Are you talking about the drive gears like what you would see in a TH350? Cause I don't think that will work as the speedo is cable driven on those cars.

I thought the speedo is set electronically and only a PCM tune or some type of electronic device put in series of the VSS would recalibrate for a gear change affecting the speedo. Maybe what you were linking was for a device like that.
 

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The 4L60 has an electronic VSS that has a gear that mounts on a shaft with a flat for a key. The gear does not have a shaft like the mechanical drives. The driven gear is held on the output shaft with a clip, and a slight interference fit. It must be removed with some pressure. The drive gear comes in three sizes 15, 17, and 18 tooth. The driven gear comes in sizes from 34-45 teeth. Few gears are readily available. There are also two different VSS units. One for up to and including 39 teeth, and one for 40 teeth and over.

I am including a picture of a driven gear.
 

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Finally, chapter 3 of my rear end saga. My Roadmaster had a 2.56 open rear-end, I bought an SS 3.42 posi off Craigslist. Installed and ran it for a few weeks then the pinion nut backed off and I re-installed the original. Had it rebuilt again with a 3.73 gear and installed today. Took me about 4 hours to do it on the ground, I am 58 years old, 6'2" and a couple biscuits shy of 300 lbs. I WILL be sore tomorrow!! I pulled it out of the drive to repark it in my drive and ran across my neighbor in the cul de sac, he encouraged me to light them up and I did. Still need to re-bleed the brakes and install the SS parking brake cables and re-calibrate the speedometer. Got a Dakota Digital unit to do the speedo. BMR upper and lower control arms, BMR 1.5" rear sway bar, Spohn rear shock re-location brackets, Eibach springs, Monroe super duty shocks.
 

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