Gear RatioExplanation Wanted

[kgelliott]

I thought I had a basic understanding of this, but it seems I don't. When I have looked at the specs for C6's transmissions, it shows the gear ratios for the M6 trans. and the A6 trans:

6 Man 6 Auto
1st 2.66 4.02
2nd 1.78 2.36
3rd 1.30 1.53
4th 1.00 1.15
5th 0.74 0.85
6th 0.50 0.67

R 2.90 3.06

Final Drive
Ratio 3.42 2.56


Can you give a basic explanation for these numbers, what they mean in terms of performance, and why the differences between the Man. and Auto? It seems like a big difference for 1st gear. Does this mean an A6 would be quicker the first hundred feet than a M6? What about 6th gear, if both have the same top end speed capability?
Thanks for helpfulness.

 

[DRTH VTR]

I think that the transmission ratio describes how many turns of the input shaft result in one turn of the output shaft. The final drive looks like it represents the ratio at the differential.

1st gear auto 4.02 X 2.56 = 10.3 revolutions of the input shaft per revolution of the rear tires.

1st gear manual 2.66 X 3.42 = 9.1 revolutions of the input shaft per revolution of the rear tires.

6th gear auto 0.67 X 2.56 = 1.7
6th gear manual 0.5 X 3.42 = 1.7

the end result is fairly similar; then there is the torque converter as another variable.

If this is wrong, somebody please correct me.

 

[kgelliott]

Hope you don't mind the questions. Thanks for the explanation so far!! I figure this is the place to learn.
Why would the manual tranmission ratios be different than the automatic's? Also, since there is a big 1st gear differnece, does that translate into a performance difference?

 

[DRTH VTR]

First, I am NOT an expert. I post my own understanding of the topic. I hope an expert will come along and give the real scoop.

As far as the manual vs the automatic goes- There is one thing that we have left out. That is the torque converter. When starting out in first, there is some slippage within the torque converter. Then there is the concept of torque multiplication, which I do not understand very well at all.

I think that the gear ratios and differential gearing was well thought out by the GM engineers. They wanted to find a balance between performance and economy. They did not want to be required to pay the gas guzzler tax. But you can change all that if you want to. Going to a higher gear ratio in the differential would likely improve acceleration, but at the expense of top speed and fuel economy. If I had a fatter wallet, I would do exactly that with my Z06.