cliffyk

The stock gears on your '04 are 3.27:1.

I have 3.73s and get just about the same mileage as you describe, however that is with a tune optimized for 93 octane fuel. Before I got the 3.73's, but had a pretty good tune, I actually got 1 to 1-1/2 mpg better than with the stock tune--the 3.73s cost me that...

Z28KLR

I guess all I can say is that everybody's car and more importantly, driving habits are different. I lost about 2-3 mpg overall with gears, but not everybody does. If I can spend and entire tank on the freeway (I usually keep it around 80-85 mph) I could expect maybe 20-22 mpg at best, but the 18.3 mpg I noted in my earlier post is pretty average and mostly what I'm used to getting.

ValeTudo

Here is a video of the sound if it is helpful in diagnosing the problem:

http://www.youtube.com/watch?v=_TkE7kC0sMw