regorih

I was wondering if anyone has ever put the 5th and 6th gear of a M6(C5) into a M12(C5zo6) to get better highway millage??

Roger

obi-1

Good question. I was wondering the same thing. Or if it is possible to put the M12 1-3 into the M6?

Fastbird

If you're having a transmission rebuilt, I don't see why not. Haven't specifically heard of anyone doing that though.

AllVette

The 5th and 6th gears on the M12 and the M6 are the same parts. The difference in ratio comes from the different input shaft gear and its counterpart on the cluster gear (counter gear.)

I believe you can get aftermarket gears to change 5th and 6th ratios without disturbing 1-4.

ragtopws6

Correct! The 5th/6th gears are the same on both trans and the difference in the 2 overdrives comes from the gear reduction between the input (4h gear) gear and the primary cluster gear.

Contact RPM Transmissions. Rodney can help you select a custom over drive ratios!

obi-1

Good info. I didn't know it was possible to have custom gears made for the trans.

0TJ@RPMtransmissions

Give us a call we can help you out. 1-800-406-1109

AU N EGL

Been thinking about taking the 5th gear out of the M12 and put into my M6.

Then my M6 5th gear ratio as my M6 6th gear.

Then close to if not something like this:

1st gear ratio 2.66
2nd gear ratio 1.78
3rd gear ratio 1.3
4th gear ratio 1
5th gear ratio 0.84
6th gear ratio 0.74


or just go to this:

1st gear ratio 2.29
2nd gear ratio 1.6
3rd gear ratio 1.21
4th gear ratio 1
5th gear ratio 0.85
6th gear ratio 0.76

Stock MN6
1st gear ratio 2.66
2nd gear ratio 1.78
3rd gear ratio 1.3
4th gear ratio 1
5th gear ratio 0.74
6th gear ratio 0.5

Stock M12 ratio
1st gear ratio 2.97
2nd gear ratio 2.07
3rd gear ratio 1.43
4th gear ratio 1
5th gear ratio 0.84
6th gear ratio 0.56

JVetthead

Thats what i did... Put Zo6 5th and 6th in my m6 box.
m6 1-4 and m12 5-6 makes for closer ratios for road course.
And no you dont have to have them made, its a bolt on

ragtopws6

Did you guys skip my post? The 5-6 is MM6 and M12 is the SAME!!!!!!!!!

JVetthead

Looks like i need to have a talk with my trans builder

ragtopws6

T56 gear counts

MM6

The gear on the input shaft is 31 teeth, it drives the counter shaft gear with 36 teeth, for a constant .86 reduction in countershaft speed.

The 1st gear counter gear is 17t, mainshaft 39t = 2.294 / .86 = 2.66
The 2nd gear counter gear is 28t, mainshaft 43t = 1.536/ .86 = 1.78
The 3rd gear counter gear is 33t, mainshaft 37t = 1.121/ .86 = 1.30
4th gear is just locking the mainshaft to the input shaft = 1.0000000
The 5th gear ext. gear is 58t, mainshaft 37t = 0.638/ .86 = 0.740
The 6th gear ext. gear is 67t, mainshaft 29t = 0.433/ .86 = 0.503

Rev gear counter is 14t, mainshaft 35t, case 23t = 2.5 / .86 = 2.90

MN12

The gear on the input shaft is 29 teeth, it drives the counter shaft gear with 38 teeth, for a constant .76 reduction in countershaft speed.

The 1st gear counter gear is 19t, mainshaft 43t = 2.263 / .76 = 2.97
The 2nd gear counter gear is 26t, mainshaft 41t = 1.577/ .76 = 2.07
The 3rd gear counter gear is 34t, mainshaft 37t = 1.088/ .76 = 1.43
4th gear is just locking the mainshaft to the input shaft = 1.0000000
The 5th gear ext. gear is 58t, mainshaft 37t = 0.638/ .76 = 0.84
The 6th gear ext. gear is 67t, mainshaft 29t = 0.433/ .76 = 0.56

Rev gear counter is 14t, mainshaft 35t, case 23t = 2.5 / .76 = 3.28

The '93 M29 5/6 gears are:

The 5th gear ext. gear is 59t, mainshaft 36t = 0.610/ .76 = 0.80
The 6th gear ext. gear is 65t, mainshaft 31t = 0.477/ .76 = 0.62

The input/counter reduction ratio is the same as the M12, so putting the '93 5/6 driven and drive gears in one would give the same .80 and .62 ODs.

Putting them in the .86 ratio MM6, would give 0.710 5th gear, and a 0.555 6th gear.

sperkins

Digging up an old thread.

So even though 5th gear in the MN6 and M12 transmissions are the same (58t), wouldn't you still have to physically change it to a different ratio to get the M12 5th gear ratio ratio in a MN6 transmission due to the difference in the countershaft speed?
A 51 tooth 5th gear in a MN6 trans would yield a ratio of .843 which is nearly identical to the stock M12 trans.

5th gear ext. gear of 51t, mainshaft 37t = 0.725/ .86 (reduction speed) = 0.843

Am I doing this right?

sperkins

ttt

cdkcorvette7

Looks right to me... Call RPM and ask them. The can get what you need too...

Rick@RKT Performance

There are few companies that do make custom gears but they are expensive. What trans builders do is to use overdrive/driven gears from other T56's (Mustang Cobra 0.79; 0.63). Only problem is the Ford T56 have 2 piece driven gears and the main shaft is splined for both gears. The Corvette T56 has 1 piece 5-6 cluster driven gears and splined only on the 5th gear side.

sperkins

So how are they doing it?
Is there no one that carries the 51 tooth gear? Seems like it'd be a popular item among the road racers.

Rick@RKT Performance

The Cobra has 54 tooth driving 37 tooth on the 5th. 61 tooth driving 33 tooth on the 6th. You need a custom main shaft that is fully splined where the over driven gears sit.

I used to have this as an option on my builds but when I tell them how much it would cost, most people pass on this option. Besides, most road racers seldom use 5th or 6th.

Bad News

Ok so the calculation for 5th isnt making any since to me. .84? Isnt a MN12's 5th gear .57?

Rick@RKT Performance

Tremec makes the 5th gears (drive gear and driven gear) as a matched set. You can't just replace 1 gear and try to make it work. Unfortunately, the Corvette T56 has a cluster (5th and 6th) driven gear that is 1 piece. In order to change the 5th gear, you have to change the 6th gear also.