Wellll...

Here we go... tons of reading on this topic... putting on my flame-prooof underoosss...

Got a got used 3-ribbed diff... sending it to be rebuilt... what rear gear????

2002 Head and Cam ZO6... trans already rebuilt... all trans ratios factory.

3:73.... OR.... 3:90 ???

 

4:11:1

 

With the Z06 trans gearing 3.90/4.10 might be a little much. Theres a good chart somewhere on here showing the rpm/mph in each gear with different gear ratios you should check out

 

What's your goal with the car?

 

Seek and you shall find. I agree with cautioning the 3.90 or 4.10 as they could be a bit much with the M12 trans. I sure do love a 4.10 in an NA C5 with an MM6. So I vote 3.73 with the M12 which is between 100 to 150rpm higher than the MM6 w/4.10 across all 6 gears. And if you decide to boost it for big power you're more likely to regret going lower than 3.73 with the M12.

 

I vote 3.42 or 3.73 honestly. Unless you plan to drag race the thing, a little less is more. More gear is fun, but can come at the cost of traction in lower gears, which is also fun and keeps us out of trouble. I would rather be able to completely hook in 2nd with less gear than hooking only in 3rd and up with more gear. If you aren't drag racing the car and don't have infinite amounts of grip, it just shifts the gear and speed higher where you have traction and increases the likelihood of a ticket if caught.

Been there, done that mod many times. As an occasional drag racer with no regard to handling and running around 24/7 on an aggressive drag radial, I loved steep gearing (gearing the car for the 1/4). I'm still tempted to switch the 3.73's in my Z06 when a clutch goes in, but I will likely follow my own advice and leave it be with the 3.42's.

My $0.02

 

The lower gears are a blast and play really nice with cams.


If you do a lot of freeway, then 3.42

If mostly surface streets and some freeway, then 3.73 is probably the sweet spot. ✅

I feel the 3.90’s would be annoying around town after a while.

Ps. I have the mm6 with 4.10’s. Im driving surface streets 97.3754% of the time. And I LOVE these gears. In normal traffic I often skip 3rd, and at greater than 45mph I often shift into 6th.

With the closer ranges in the Z, I suspect a bit higher gear would be easier to drive in your car than mine, so there’s more to this than just high gear cruising rpm’s. Probably better to err on the side of a higher gear with your trans ratios… 3.73 seems like a good overall choice.

 

My N/A C5Z made right at 450whp and with a 3.90 rear, it was a blast to drive. I've considered installing that gear in my Vert, with MM6.
As expensive as a gear swap is on one of these cars, going from a 3.42 to a 3.73 almost seems like a waste of money, but that's only my opinion.

 

similar logic has kept me mostly convinced to stick with the 3.42 diff I've had in my car the last 5 years. I ran a 4.10 for 15+ years on two different H/C combos. I also ran 3.90 on another coupe I had. High revving cams with more rpm to play with pair perfectly with a lower gear. Frankly I absolutely HATE 3.42 gearing in a naturally aspirated C5 with M6 trans. The M12/3.42 having 3.90 equivalent is almost perfect for NA builds.

Where my experience differs from the above...I've only owned M6 T56 cars and find the 3.42s lazy AF in low rpms and much more difficult to find traction off the line (with street tires or a DR) compared to both 3.90 and 4.10 with 420 to 430whp. The 3.42 takes a lot more clutch modulation and slipping to find the sweet spot (for me anyway) which requires a lot more practice to not bog or wheel spin out of the hole. The steeper gears make it much easier to launch without bog or excessive spinning. And with the steeper gearset the low speed parking lot and neighborhood driving is much more pleasant compared to the 3.42 that wants more rpm. The 3.42 has you hunting for the sweet spot when lugging around slowly.

Now to play devil's advocate against the lower gear...with H/C/FI when doing WOT pulls you're blowing through RPMs with a major quickness. The longer gearing is most favorable under WOT with boost. So it pays to know early on whether your intentions will be to go forced induction and whether or not you'll spend a lot of time chasing your PB at the dragstrip with the car.

I run around on NT555R2's and find them very adequate for street driving and traction with 640whp/568wtq. With the right tire I don't see traction being an issue if you go with a lower gear on an NA build and don't treat your throttle like an on/off switch. The dislike & disadvantages will be with the shortened shift times as your power to the wheels goes up. Which is why my 3.90 diff is still sitting on a shelf in my garage. 80% of the time I hate the 3.42 but I start to really appreciate it when I'm ringing the gears out.

If you stick with a normal street tire vs. a DR a 400whp C5 with a clutch and 3.42 can & will blow the tires off on the line and 1-2 shifts. The power and the gear isn't the issue under 500whp. It's the tires, suspension and driver technique.

It's nice having the option to swap the diff in and out from 3.42 to 3.73, 3.90, or 4.10.
Lower the gear...get a better tire... make suspension improvements...and practice your launches with the lower gear. Or just say F it...do the gear and get a WOT box with 2 stage!

 

I agree with this sentiment/perspective as well. Lol! After the fact the 3.73 may not be noticeable enough after the investment. And you'll wish you went to a 3.90 instead. If the cam of choice makes for another 400 to 600 rpm between shifts the 3.90 with the M12 would be fun and a noticeable upgrade.

 

Im confused. You use 2nd gear when driving around??? Don't go 1 to 4???? The only time i use 2nd is if i have to race if not, im 1 to 4 to 6 since i had the car.

 

Some of us have gigantic cams (you know, compensation...) and can't lug the motor

I don't use 6th with C5Z gearing until 80mph, and I run every gear to 4k before shifting to keep my motor in its happy spot when grabbing the next gear

I'll be out there in 3rd at 45mph lmao

 

Maybe you’re easily confused?

 

Is this real? Did you keep the CAGS installed on your car? Worst option ever in a sports car. I had mine tuned out right after my warranty ended in 2003. Lol!

With a decent cam 1 to 4 shifts (even around town) offers a very impotent driving experience. I may 1-4 here and there but not on purpose. Lol!

 

I do it for pain management. I don't know, i shift before 2k so 1st to 4th them 6th. If i'm on a 25mph road i'll be in 5th at 30mph if not im in 6th going over 35. so i only shift twice with every take off. I should've mentioned when i push the clutch pedal in i get lighting bolts up my left leg. Been like this for yrs. As i mentioned in prior comments i can not drive my car anymore i got to the point where it just wasn't worth the pain to drive. I have to get 4 discs fused on my lower back before i can even attempt to drive my c5 again and my shifter arm is ***'ed. Just got out of surgery for my left arm and know im going back in in 2 weeks for my right arm then my lower back then my neck and maybe ill be able to drive my car again. if the surgeries dont work it will just be a huge paper weight. My car is close 600qt by 3k 6psi of boost so it has no problem with bogging. Plenty of tq. It's hard to explain the power when ur will over 1krwhp. My car was set up for light to light race if anybody wants to try their luck. my cam is 111 LS over 10.25 compression. I would of taken a video to explain. I know its hard to believer but im at 28psi of boost at 6.5k. with my pulley comdo i can hit 32 by upping the redline but wanted to keep it safe because im done working on this car if i can ever drive it again. Im about 8 or 900 rpm cursing at 35 in 6th. My car dont surg in any rpm i tune my car myself so putting hrs after hrs and days to get the drivability to perfection which most tuner shops don't do. If they did your tune would cost thousands of dollars on the time to get it perfect in all rpm range. Maybe someday i'll get a vid. You will never understand how much Tq is in this car down low. Most or all big HP car don't feel the power until upper rpm range with f-1x its all over the rpm range. Just dont need to be above 1k pudding around town if it's not surging. And probably saved 100s or thousands of shift. You don't realise how many shifts to get your car up to speed when you don't feel lighting bolts up your leg with every shift. Try sticking a knife in your lower back and go out for a ride and let me know how you feel shifting 5 time compared to 2 times driving around to work. This was my daily driver so i went everywhere with it.

 

Man, that sucks about your health. I'm sorry to hear you're struggling through all that. I hope things shape up for you post surgeries. A powerglide swap is an alternative but at $10k for the trans or $16k+ for the trans & conversion installed it's not a low cost alternative. But if your love and passion is in your particular car it could be the right choce if budget permits. Life's too short perspective..

To your point, if you're car is making massive power skipping gears to cruise around town is more of a requirement than a choice. My car drives 100% like stock under 3k rpms. Which is why I'm not a fan of the 1-4 shifts even when leaving my 25mph zone neighborhood. I absolutely hated the CAGs for that very same reason.

I also do NOT like lunking around in a gear that doesn't allow immediate throttle response in the case I need to make a defensive driving decision/move on demand. Nothing worse than the need to accelerate being handicapped by a lazy gear. Requiring a shift to avoid these scenarios can be dangerous. Sort of like how much I despise driving my wife's Wrangler Cheif. Stab the gas and mostly just make noise without much forward momentum.

 

LOL "Stab the gas and mostly just make noise without much forward momentum."
My car i drive now is a 2010 kia rio when the wife has here Blazer RS, so when it's available i drive the RS because you go up to get in not down, the Kia is neutral so not that bad.. I don't even try to pass someone in the kia. but the avage 35 mile to the gallons is a lot better then 8 mile to gallon with the vet.

 

Honestly, I just needed to Google the Kia Rio. I didn't know anything about them. My 1985 Yamaha motorcycle makes more hp than a Rio. Lol! Talk about going from one extreme to the other. Good luck with getting yourself back to enjoying your C5. Where you belong!

OP...either the 3.73 or the 3.90...even a 2010 Kia Rio has a 4.06 final drive ratio!

 

This comment coming from a well known forum member, you might want to consider a 3.90. I bought my stage 2 diff from RPM transmission, maybe consider them for a rebuild.

 

My coupe has had 4.10 gears since 2008. I only have basic bolt-ons but it is lots of fun with the gears.