I have 3.91 gears in my GTO and they are great 1st through 5th, but cruising rpms on the highway went up by a few hundred. While my trans is out getting a new clutch and an MGW-P, I was thinking having a .50 6th gear would be nice as opposed to the .56 stock gear. It's not much, but if it is not a complicated swap, it might be worthwhile to get cruising rpms back down where they used to be.

Is this possible? If so, how much should I expect to pay for the parts and where would I find them?

 

Not easily done. The GTO (M12) 5th/6th gears are identical to the Fbody,etc. (MM6) gears. The M12 5/6 final ratios are different because of the input/counter ratio differences.

 

As Mike said, the gears between the two share the same tooth counts. What changes the ratio of the 6th gear is the input shaft and cluster gear tooth counts which are different between the 2.66 and 2.97 ratios.

 

So it would require a new input shaft and counter at a minimum.

 

A complete MM6 gear set (C5 corvette), or finding an old 1993 M28/M29 5th/6th driven/drive gear set, or a full custom gearset..

As I said, not easy, nor cheap.

 

And on top of all this, I would have no choice but to change my 5th gear as well?

 

The 5/6 driven and the 5th and 6th syncro'd gears are the same. But by nature of changing the input shaft to countershaft ratio....all output gears would change. 1-6 and rev.

That 2.97 first with a 3.91 rear must be just stupid hard to get traction. I have a 2.66 with a 4.1 and want to go to a 3.73 granted my car only weighs 3k.

I've always thought that a car with the 5th gear from a cobra with the 6th from an f-body would be an awesome car for the street/strip. But until someone makes a custom 5/6 driven it's not going to happen.

 

In theory, yes. Since the cluster gear is one piece and the tooth counts are different for 1-2-3 as well, you would have to replace the 1st gear, 2nd gear, 3rd gear, input and cluster gear so everything matches.