: Is anyone running 3.91:1 rear gearing...

05-13-06, 12:18 PM
I noticed that the getrag 645 went up to 3.91:1 (as posted on their website). http://www.getrag.com/181/181

Has anyone installed the 3.91:1 rear gearing... If so, was it worth it, and how much did it lower your 1/4 mile time by. I am considering on this done soon.

Also.... What was the cost, start-finish w/parts & labor. Thanks. :)

05-13-06, 01:08 PM
arent we all running 3.91:1 in the rear? Thought it was the standard ratio.


05-13-06, 01:13 PM
We're running 3.73 gears.

05-13-06, 01:15 PM
F -

per the .faq it is a 3.73


05-13-06, 01:32 PM
The CTS-V comes with 3.73 gears. Can't get any traction in the quarter with the stock ratio. End up having to start from second gear. Wouldn't want to make matters worse. :)


05-13-06, 02:02 PM
my bad....thanks fellas.


05-13-06, 05:02 PM
Parkers diff will be 3.90's when it's done.

05-13-06, 07:50 PM
3.73 is the ratio that we generally would want, especially for the street and with the trans that comes with the caddy. It would be a rare combination that would want a higher ratio. :)

05-13-06, 07:54 PM
Good point - with a 245/40-18 tire, the effective gear ratio goes from a 3.73:1 to 3.87:1. There's a "track star" V owner on the forums running 255/35-18s on road courses for an effective gear of 3.98:1.

Track or street, more gear is fun.

you can run an effective gear by changing to a small diameter tire... the red line comes up really fast with smaller tires...

05-13-06, 10:43 PM
Keep in mind the V uses the shorter Z06 spec "M12" T56 trans.

The ratios are shorter than in the MN6 T56 used in the F-Body and standard C5 / C6. The F-Bod used a 3.23 rear. The C5/C6 use a 3.42 rear.

The GTO uses the M12 tranny as well, but with 3.46 rear gears.

So, the V has the shortest gears in the GM stable combining the M12 and the 3.73. The 3.91 are probably not worth the effort.

V Amazed
05-13-06, 11:05 PM
export model has a 3.91 rear, and if you all caught it, on the top gear video, Jeremy Clarkson said it did 0-60 in 4.3


05-14-06, 03:51 AM
The 3.91 are probably not worth the effort.05-13-06 04:54 PM
I agree, I'd want to go right to 4.10's. The first two gears are fine, but this heavy car wants more gear on the street for everything but launch. My 550 hp 2003 Mustang Cobra ran 4.10's and it was the perfect gear.

As soon as I can afford that Ford 9" it will go in with a 4.10 (4.11, whatever).

05-14-06, 10:48 AM
As soon as I can afford that Ford 9" it will go in with a 4.10 (4.11, whatever).

That is not a bad idea...

My assessment was also based on the reality of seeing my just installed 3.91s bathed in gear oil, laying on the side of the road somewhere, Diff carrier split on half like a little walnut.

If anyone is serious about installing gears - they shouldn't even think about touching the stock diff. It won't hold up to more than 5 launches...at best.