Change the gear ratio

Hi folks

I would like to change my gear ratio on my Buick GS 400 with the 12 Bolt axle. I determine the current ratio to 3.08 (1 Full Tire turn = 3 and a little on the drive shaft).

Target ratio should be 3.42 or 3.73 .

Transmission will be a T56 Magnum with :[TABLE="width: 80%"]
<tbody>[TR]
[TD][/TD]
[TD]1st[/TD]
[TD]2nd[/TD]
[TD]3rd[/TD]
[TD]4th[/TD]
[TD]5th[/TD]
[TD]6th[/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD]2.66[/TD]
[TD]1.78[/TD]
[TD]1.30[/TD]
[TD]1.00[/TD]
[TD]0.80[/TD]
[TD]0.63[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

There are a lot ring and pinion gear sets for GM but which is the right for the Buick? and what brand i should take?

Any suggetions and advices are great

Kind Regards

Robert

 

Canadian built GS's do have 12 bolt rears. I like the 355's myself.

 

My car was built in Ottawa and was an export model.



The cover has 12 Bolt, that's reason why i guess its a 12 bolt

 

It is the bolt count on the ring gear that determines 12 vs 10. The cover bolt count is a misconception. (Not trying to be a smart azz here). Mike D.

 

Remove the cover, take a picture of the inside of the rear end and send it to Monzaz here on the board... he will help you ID it and get you the parts you need.

 

Put 4.10 in it. You'll love it. Plenty of grunt and torque real fun on the street

 

He could go with 4.10 if he wanted, judging by the .63 overdrive ratio:grin:

 

X2. I've got 200-4R with a .67 OD and 4.10 gears. Great 1st gear and 70mph@2,400rpm.

 

Likewise. I run a 3.89 gear in my Skylark with a .64 overdrive. I picked that gear based on what I expect to mph/redline in the 1/4. I get around 19-19.5mpg with the combination.

I run 4.56 gears and .67 OD (200-4r) in my Firebird. Makes the 250cid L6 quite peppy in 1st and 2nd p The small engine still gets 20mpg on the highway.

 

Okay you have convinced me to take a higher gear. Important for me is a good performace / mpg ratio. A gallon gas cost here $6.60 and we don't have an 1/4 Mile track. I just want cruise around the town and drive on highway's for visit a US car meeting. I guess the 3.73 gears do the right job. Before i order the gear, i will take the cover off and inspect the ring gear to determine how much bolts it has. I saw the axle has stamped numbers, can these verify the kind of it? Or Larry are you sure about its a 12 bolt? MikeD unsettled me a bit...

 

That looks like the same rear end I have in my GS ( Chevy 12 bolt posi I got for free) with C-clip axles.
When you order gears, bearings, shims and stuff, make sure you order it for a 12 bolt car, NOT 12 bolt truck.

 

My tires are 26.6" tall and i need your advice again. The transmission is available in in close and wide ratios

Here is a link with a compare: Close ratio with 3.73 rear gear and wide ratio with 4.11 rear gear

What are your thoughts?

 

I did write with Jim from hurst driveline. He's the tech guy and he recommend me to use the close ratio and 3.73 Gears. I do some math and he is absolute right. The critical spot is the speed of the drive shaft. My car must ( not my opinion, its the Law here) run the same maximum speed as before. The speed is 125 mph and with the 4.11 and a 27" tire my drive shaft turns 6400 RPMs. The 3.73 gears will reduce the speed to 5800 RPM. I asked him about the max RPM of the common drive shafts:

A 55 inch steel drive shaft 3-1/2 inches in diameter has a critical speed of 5634 RPM
A 55 inch steel drive shaft 4 inches in diameter has a critical speed of 6423 RPM
A 55 inch Aluminum drive shaft 3-1/2 inches in diameter has a critical speed of 6381 RPM
A 55 inch Aluminum drive shaft 4 inches in diameter has a critical speed of 7245 RPM

I will buy the 3 1/2 aluminum drive shaft for comfort and safety.

Larry are these gears the right ones? Is the summit stuff good or should i take a better brand?

Thanks for your help

Robert

 

Larry

It will be a fun car. cruising around with hard pull's sometimes