I've got an 02 Z06 and an 02 Coupe. Both exhibit different RPMs at highway speeds. The coupe at 75 shows about 1750 and the Z shows about 1700 at 65. Basically, I've seen that the Z is shorter geared than the coupe at all speeds.

Did GM use different gearing in the Z cars? If so, then what is it?

Thanks!

 

All MN6 cars all came with 3:42s. The Z06 is therefore equipped with 3:42s.
All the automatics came with either a 2:73 or 3:15s.

Another thing at play here is the fact that the MN6s' 6th gear (which the auto does not have of course) is a .5:1 ratio and will drop the rpms lower than the auto.

HTH

 

Yes, the gearing in the transmission is different in the Z06. There is a direct comparison on the corvette action center website.

http://www.corvetteactioncenter.com/...2001specs.html

 

The Z does have different gearing in the lower gears than the "regular" 6 spds, but once in top gear, the ratios are the same, regardless if it's a Z or a non-Z.
The "overdrive" ratio of .5 to 1 in 6th gear is the main reason for the lower rpms at highway cruise speed (with both transmissions in "top" gear).
The difference in rpms in "top" gear tells me the '02 coupe is an automatic.

 

Well I'll have to disagree with you on that. Both cars have different RPMs in the top gear (like I had mentioned in my first post as I don't cruise on a highway in any other gear than 6th). Both cars are also M6 cars.

What gives?

 

You're right. I just re-read your post and see you are comparing the rpms with one car at 65 mph and the other car at 75 mph. That little "minor" fact will easily account for the difference.

Bottom line: The Z has no "unique" or "different" gear ratios once in 6th gear. Each car (in stock form) has the same rear gear (3.42)and the same final .5 to 1 drive ratio in 6th gear.
Compare the rpms at the same speed.

 

Huh? What are you talking about? lol 10 mph is not going to be a diff of 50 rpms. Final gear ratios are still different...either coming from the rear or the tranny. I've got the RPMs to prove it!

 

If you have the original factory set-up on both cars, the ratios are the same for both cars, both rear end AND transmission (in 6th gear). The only difference between the two is the Z has different gearing in the lower gearing as you say.
Have you actually tried running both cars cars at the same speed and then compared the rpms? Within a small margin of error, ie. slightly different tire diameter due to wear, or small variations on speedo accuracy, they will be the same.

 

The Coupe 6th gear ratio is 0.50:1 whereas the Z with the MN12 6th gear is 0.56:1 ratio. Also, the coupe rear tires are 275/40-18 (26.7" overall height) compared to the Z rear tires which are 295/35-18 (26.1" overall height).

It is also possible your tach is off a little, as 'theoretical' rpm at 75mph with the MN6 car (coupe) is ~1615rpm and the 'theoretical rpm at 65mph with the MN12 car (Z06) is ~1603rpm.

So in short, the slightly different ratio will account for some of that difference, but tire height will play a role also.

 

You make a good point. The final (6th gear) numbers do vary by .06 on the Z. Good catch.
Either way, a comparison needs to be made at the same speeds in order to get a close to accurate number.

 

Go to the website on my post above and compare the final drive ratio yourself.

 

Coupe Redline is ~6200, Z06 redline is 6500 in stock configuration

Gear Ratios - Only 4th gear 1:1 is the same

MN6 ( Coupe)
2.66
1.78
1.3
1
0.74
0.5

C5 MN12 ( Z06)
2.97
2.07
1.43
1
0.84
0.56

C6

MZ6
2.97
2.07
1.43
1
0.71
0.57

C6 gear w/o Z51
2.66
1.78
1.3
1
0.74
0.5

TR-6060
2.97
1.78
1.3
1
0.8
0.63

 

Correct. See post #10.

BTW, FWIW I show on mine:

65 = 1450
75 = 1650
85 = 1850

 

He's 100% right. Both cars are running through the same 3.42 rear end, and thus will be at different rpm's at the same speed in any gear EXCEPT 4th.

As far as 6th gear, based on the numbers published by GM for the two cars, the Z06 would be going 81.2mph at 2,000rpm's, while the coupe with an MN6 would be going 92.77mph at 2,000rpm's. That little .06 difference in 6th gear makes a significant difference. 5th gear at 2,000 rpm is 54.1mph (Z06) vs 62.7mph (Coupe).

Those numbers come off of a spreadsheet I have in Excel, which gives the following rpm numbers for the speeds talonhead posted: Coupe at 75mph = 1,615rpm. Z06 at 70mph = 1,600rpm. Keep in mind that tire wear and probably tire pressure could effect this (the spreadsheet takes into account the tire width and profile% to calculate total tire diameter), and also keep in mind that the stock tachometer could very well be off a couple hundred rpm. I know when my FRC was stock the rev limiter seemed to jump in at 6,000 or just a hair past, but well below an indicated 6,200rpm.

 

Make sure you account for tire size differences also.

This also means they are not at the same rpm/mph in 4th gear even despite the same final gear ratio between the two.

 

Yep.

But two different things are being discussed here... the transmission gear ratios, and the rear axle ratio . The rear axle ratio is the same for all 6 speed C5's.

But the Z06 Tranny does have different internal gear ratios than the non Z06 manual trannys.

I have these listed at the bottom of my web page:

http://mikemercury.home.att.net/ratio.htm

 

You are correct. In 4th gear at 2,000 rpm's, the speed difference is about 1mph. 46.4mph in the coupe, and 45.46mph in the Z. The Z's tires are 35's instead of 40's, and thus will have pretty darned close to the same overall diameter. The sidewall is 35% of 295mm = 103.25 vs. 40% of 275 = 110. This would be about a half of an inch of total diameter difference.

 

Here ya go. My Z at 1900 at 75. My coupe is at about 1700-1750 going 75.

Now to me, that is not from a diff in the tire size, tread depth, or the tranny's .06 difference. It's just too much of RPM difference to me.

Any players?

 

The coupe at 75mph should read 1615rpm, as stated above. At 75mph, the Z should read 1850rpm.


Trust me, there is enough difference there for a 235rpm difference.

Your speedometer doesn't change as your tires wear down, it is simply based on axle revolutions vs a theoretical tire size so the calculated numbers will be correct, its your car (tach/speedometer) that is off.

100rpm or 1mph resolution would be very accurate for something like a tach or speedometer, think of the angular precision necessary for that when the indicating arm is a few inches away from the axis of rotation.

 

Here are the gear ratios in the MN6 and MN12 transmissions:
------MN6 -- MN12 -Custom MN6
`1st - 2.66 - 2.97 - 2.66
2nd - 1.78 - 2.07 - 1.78
3rd - 1.30 - 1.43 - 1.30
4th - 1.00 - 1.00 - 1.00
5th - 0.74 - 0.84 - 0.74
6th - 0.50 - 0.56 - 0.563

I swapped the MN12 in my Z06 for a MN6 and had 6th gear changed to 0.563:1 like it is in the MN12 trans. With my 4.10:1 final drive ratio I have a very driveable 6th gear.

Here is a graph that shows the RPM loss after a 6,500 rpm shift for all three transmissions:




Here is another graph that shows the RPM loss after a 6,500 rpm shift with a close-ratio MN6, where 5th gear is 0.80:1 and 6th gear is 0.70:1: