3.90 Rear End Gears
Thread Starter
Le Mans Master
Joined: Dec 2000
Posts: 5,245
Likes: 25
From: Blue Ridge Georgia
3.90 Rear End Gears
I'm starting to plan my next mod and I'm thinking about changing out the 3.42's in my Z to 3.90 gears. I really only drive around town so I think it will make 5th & 6th gears more useable, plus a little more fun overall. I don't mind shifting so if the shifting increases it's no big deal. Who has changed from 3.42 to 3.90 and how do you like it?
Burning Brakes
Joined: Jun 2010
Posts: 769
Likes: 1
From: Columbia TN
I did the change and I've really enjoyed it. I had it done the same time I had a cam installed so it's kind of hard to say how much it changed because of the cam. Now I'm in the middle of a stroker build and I'm going to keep the gears.
Go for it.
Go for it.
Drifting
Joined: Apr 2009
Posts: 1,898
Likes: 9
From: Morrisville PA
I personally think for a c5z, it's pointless unless it's seeing the 1/4 mile on sticky tires frequently.
Gearing a car has diminishing returns just like anything else, and the c5z ratios are already like a base C5 with 3.90s 1-3.
Assuming stock red line of 6500, your first gear becomes 45 mph, your 2nd gear becomes 65 mph, and your 3rd gear only becomes 95 mph.
Where you will gain is acceleration in 4th gear.
Gearing a car has diminishing returns just like anything else, and the c5z ratios are already like a base C5 with 3.90s 1-3.
Assuming stock red line of 6500, your first gear becomes 45 mph, your 2nd gear becomes 65 mph, and your 3rd gear only becomes 95 mph.
Where you will gain is acceleration in 4th gear.
Race Director
Joined: Sep 2005
Posts: 11,060
Likes: 291
From: Pasco WA
I wouldn't do it unless you drag race alot. First gear in a M12 is 2.97:1, and with stock rear gears, is roughly equivalent to 4.30s in an old school 4 speed. The trouble with GM's choice of trans gear ratios,(nowadays) is they kept to the commonly used 4th gear being 1:1, and both 5 and 6th are OD. For street driving, all you need to do is not use 6th except on the freeway. Just my opinion, but when I went to 3.90s in my street driven Cobra, first gear was gone so fast, it was almost a waste of time, always shifting. IIRC, the old 4 spds were about 2.20 first gear. Worked great for street driving, but a change to 4.10s really made highway driving a chore w/o any OD. I guess it all comes down to racing; if you are near redline at the traps, leave it be. If you are just switching gears before the traps, go higher.
Burning Brakes
Joined: Jan 2004
Posts: 1,158
Likes: 6
From: Winter Springs Fla
I just put 410's in my c6-z06 and it made the car drive better around town and at the 1/4 mile track .
I can skip shift 1-3-5 or 2-4-5 to 50mph without lugging the motor .
Now i can actually use 6th gear at 60-65mph , with the 342 gear i couldnt use 6th below 80mph because of the drone and rpm's .
The biggest noticable difference is the exahust drone is no longer an issue around town .
By changing the gear I did go from 15 down to 13 mpg city @ 550 rwhp with a cammed 440 stroker motor.
I should be able to run 10.20's to 10.40's now on motor and drive the car everyday and still get 24mpg@80mph :P
I can skip shift 1-3-5 or 2-4-5 to 50mph without lugging the motor .
Now i can actually use 6th gear at 60-65mph , with the 342 gear i couldnt use 6th below 80mph because of the drone and rpm's .
The biggest noticable difference is the exahust drone is no longer an issue around town .
By changing the gear I did go from 15 down to 13 mpg city @ 550 rwhp with a cammed 440 stroker motor.
I should be able to run 10.20's to 10.40's now on motor and drive the car everyday and still get 24mpg@80mph :P
Former Vendor
Joined: Oct 2006
Posts: 278
Likes: 17
From: Auburn/Dacula/Lawrenceville Georgia
OP, gears are one of the single best modifications you can do to a stock geared LSX powered vehicle. The 3.91's in a Z06 is the sweet spot and makes 5th/6th gear usable around town. You can cruise in 6th @ 40mph with no problems.
Shameless plug. For those anywhere near the Atlanta area thinking about doing gears, we install them IN HOUSE. No diff shipping to anywhere, no additional down time, no core charges.
Thread Starter
Le Mans Master
Joined: Dec 2000
Posts: 5,245
Likes: 25
From: Blue Ridge Georgia
I don't make any long distance trips. Two to three hour drive would be the max. Around town if I go to 5th gear the rpm is too low. I think that the 3.90's might make town driving better. I don't race the car, I'm only a spirited driver. Most of my driving is between 45 & 60 mph. I always give the Z a Xmas gift, gears and seat covers might be this years choice.
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Former Vendor
Joined: Jan 2011
Posts: 720
Likes: 13
From: Fort Wayne Indiana
Call DTE. Gears are what we do!
I'm starting to plan my next mod and I'm thinking about changing out the 3.42's in my Z to 3.90 gears. I really only drive around town so I think it will make 5th & 6th gears more useable, plus a little more fun overall. I don't mind shifting so if the shifting increases it's no big deal. Who has changed from 3.42 to 3.90 and how do you like it?
Instructor
Joined: Feb 2006
Posts: 210
Likes: 8
From: Romeoville IL
I just switched to 3.73 gears on my coupe, I finished with it 2 weeks ago. But I did it because I had whine and a leaky butt. I am much happier with the gears. I can actually shift into 6th at 60 MPH and 4th gear is much more fun! I was torn between 3.90 and 3.73 but I went with 3.73 because I do a lot of highway driving. I would think 3.90 would be a great upgrade for you as you do a lot of city driving.
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
I have 3.90's in my vert. It really makes fifth gear more usable and sixth gear is much much better. That said, not sure how the lower gearbox ratio's will be with the lower rear as I have the standard MN6. For me, this was a great upgrade.
Le Mans Master
Joined: Jun 2002
Posts: 8,352
Likes: 103
From: In the pit, trying to love someone.
That's great, offer no help or insight whatsoever, just plug yourself for some work. Phil @ DTE was never like this.
I love what this forum is coming to.
To the OP... I have a base MN6 and had 3.90's. I loved them, and I eventually went with 4.10's. Definitely makes 5th and 6th gear usable. I go to Canada a lot, and their freeway speed limit is 100 kph which is 60 mph. With 3.42's I couldn't do that in 6th without it chugging. Now, it's a breeze.
I love what this forum is coming to.
To the OP... I have a base MN6 and had 3.90's. I loved them, and I eventually went with 4.10's. Definitely makes 5th and 6th gear usable. I go to Canada a lot, and their freeway speed limit is 100 kph which is 60 mph. With 3.42's I couldn't do that in 6th without it chugging. Now, it's a breeze.
Safety Car
Joined: Oct 2003
Posts: 3,699
Likes: 16
From: Sun City- Menifee Calif.
I'm starting to plan my next mod and I'm thinking about changing out the 3.42's in my Z to 3.90 gears. I really only drive around town so I think it will make 5th & 6th gears more useable, plus a little more fun overall. I don't mind shifting so if the shifting increases it's no big deal. Who has changed from 3.42 to 3.90 and how do you like it?
Former Vendor
Joined: Jan 2011
Posts: 720
Likes: 13
From: Fort Wayne Indiana
I do apologize for that come across as It sid. You thinking is spot on. Going to a little taller gear will make the upper gears more responsive. About all you might sacrifice is fuel economy. If you are interested numbers just look at a ratio of the ratios. Divide 3.90 by 3.42 which is 1.14. At 60 the rpm will be 1.14 times faster and the rpm required to start moving is 1.14 less. Numbers don't replace feel but it at least gives you an idea.
That's great, offer no help or insight whatsoever, just plug yourself for some work. Phil @ DTE was never like this.
I love what this forum is coming to.
To the OP... I have a base MN6 and had 3.90's. I loved them, and I eventually went with 4.10's. Definitely makes 5th and 6th gear usable. I go to Canada a lot, and their freeway speed limit is 100 kph which is 60 mph. With 3.42's I couldn't do that in 6th without it chugging. Now, it's a breeze.
I love what this forum is coming to.
To the OP... I have a base MN6 and had 3.90's. I loved them, and I eventually went with 4.10's. Definitely makes 5th and 6th gear usable. I go to Canada a lot, and their freeway speed limit is 100 kph which is 60 mph. With 3.42's I couldn't do that in 6th without it chugging. Now, it's a breeze.
Instructor
Joined: Dec 2012
Posts: 190
Likes: 3
From: Springfield Ohio
This was some good info. I have a C5 with a good size cam and it certainly is not fun driving in the city and of course 6 gear is useless unless you drive 80mph. I am thinking about going with the 3.90 as well. I wouldn't have had such a large cam installed if I was worried about gas mileage. Any thoughts on pricing? I have a couple quotes and they are around $2,000 installed, using RPM.
Burning Brakes
Joined: Feb 2012
Posts: 901
Likes: 2
From: Palm City Fl
02Z with TVS2300 /3.42s & a 99 coupe A4 headers and CAI that has 3.15s. Personally if I was going to drop $2K it would be in a cam/intake/ header upgrade (ok, maybe a little more than $2K) to give the engine more power to spinn those gears rather than change gears. Oh, I do 50mph in 6th with no problem at all. JMO
Instructor
Joined: Dec 2012
Posts: 190
Likes: 3
From: Springfield Ohio
02Z with TVS2300 /3.42s & a 99 coupe A4 headers and CAI that has 3.15s. Personally if I was going to drop $2K it would be in a cam/intake/ header upgrade (ok, maybe a little more than $2K) to give the engine more power to spinn those gears rather than change gears. Oh, I do 50mph in 6th with no problem at all. JMO
Le Mans Master
Joined: Jan 2001
Posts: 5,786
Likes: 6
From: Lafayette Colorado
Good lord. If you have a series 3 carrier, there is no need to replace the whole diff unit. Any competent mechanic some some experience can replace the gears for a couple hundred.
Last edited by kromberg; Feb 25, 2014 at 04:07 PM.