I want 3.91s
 

I'm going to buy the parts, pull the differential and take it to a drivetrain shop in town for the gear install. Help me with the logistics, please...

Who makes the best gear sets for our cars?

Besides gears, what other parts do I need to purchase? The car has 65K miles on it. I am assuming that I should at least freshen up the clutches, but I admit that I am somewhat ignorant about these rear differentials. Do they make a master install kit with bearings and clutches? If so, how much should I plan to spend on just those parts?

TIA for any help.

Janettyracing.com is who I ordered from. They have the 1LE 3.91s for like 275. Ordered them, had them in 3 days, and am totally satisfied with the results. They go well with my setup. I only needed the gears, not the full kit. I can't remember what all came in the kit, but I didn't need it, and had GPI do the gears for me. Taking a diff off is one thing, but setting the gears up is something I wanted to leave to them

Doing a full rebuild I would just go with true trac diff (no clutches) installation kit, and the gears. As said above Jannetty can supply everything. Hopefully your not planning FI...

Jannetty is doing mine this coming Tuesday. I can't wait.

Excellent, thanks for the information!

It looks like I need to visit his site... No, not going FI anytime soon. Thanks for responding.

Please let me know what you think.

why do u say that? whats FI have to do with the rear gears?

Why would you not go to 4.10's? Motive makes a good gear set that many of the fast guys are running with good success. On a stock sized tire you're still under 2000rpm at 70mph. I lost about 1-1.5mpg on the highway, and no real loss of economy at any sub-highway speeds.

It's the best mod I've ever done on my car - if you're going to go through the trouble of doing gears, why not take all you can? I did them when I was bolt on and it transformed the car. I can't imagine going with less gear than that in the M6. There isn't a downside that I see, all upsides.

With regard to the rest of the logistics you'll definitely need the full install kit. I wanna say that the ring and pinion was about $330 from motive, and maybe another $300 for the install kit.

Another thing to strongly consider is doing a different carrier - the stock posi is well known to get weak and end up a "1 tire fire" unit over time. JRE does make a kit to tighten up the clutches. Alternatively, Eaton sells the TruTrac ($610) carrier which is a gear based posi which is quite a bit stronger.

While you're beefing things up, an LPW diff ($240) cover is an option to consider. It's a cast steel piece that gives some much needed strength to the weak aluminum factory case. Additionally, it has load bolts that help hold the ring gear caps in place. JRE does make a billet set of main gear caps, but the LPW cover solves for the ring gear caps issue and helps strengthen the entire case for a similar price.

RDP has some really good black Friday promos going right now that might save you some money...

Some people think that 3.91 is overkill for FI setups, even 3.73, with stories of people roasting their tires off with all that torque, especially with the aggressive 1st gear in the 6L80. I guess I'll find out the hard way when I eventually do a Whipple and keep the 3.91s and go with a sticky tire. Am I going to break some stuff? Probably. But we are talking a 3,800 lb car with tall tires, it needs all the torque it can get.

Great point! Although a v6 runs in around 3,600 lbs. I would still get the 3.91 if you dont plan on trading up anytime soon.

Good question. It looks like I need to do some more researching. I'm interested in seeing what gear guys are trapping in with 3.91s vs. 4.10s.

4th either way. A 1LE with the MM6 trans gear ratios, stock 27.85" tall tires and 3.91 gears will go 106mph at 6500rpm in 3rd. A regular SS with M10 gear ratios, stock 28.66" tires, and 3.91 gears will go 99mph in 3rd. A bolt on M6 car will go well over 110mph, so you're going to 4th. With stock 3.45's a near stock M6 car can get it done with just third some times, but once you add some power and the trap speed climbs, you gotta make the shift.

So then, the 4.10 is the way to go without a doubt from a quarter mile perspective, you're gonna be in 4th any way you look at it even with a bolt on car, and you want all the RPM you can get in 4th. A 4.56 would even be appropriate, but nobody makes one that can live in such a small housing.

Take some time and play with gear ratios - there is a lot to learn. This is a great calculator here.

Here are the trans ratios and tire sizes:
M10 (regular SS) ratios
1st: 3.01
2nd: 2.07
3rd: 1.43
4th: 1.00
5th: 0.84
6th: 0.56
Final: 3.45
Tire: 28.66" diameter

MM6(1LE SS) ratios
1st: 2.66
2nd: 1.78
3rd: 1.30
4th: 1.00
5th: 0.74
6th: 0.50
Final: 3.91
Tire: 27.85" diameter

from a not 1/4 mile perspective, just a street car would gears still be a good upgrade?

You'll like how much lighter the car feels

Absolutely! If you're gonna do the upgrade, take it all the way!