Originally Posted by AmericanMuscleCars
Meaning what I'm not too familiar with the gear systems.
Here is the MPH for each gear at 3800 RPM for the stock 3.27 gear on a A6 V6.
At 70 MPH in 6th you are running ~1800 RPM
Now with the 3.91 gear
At 70 MPH in 6th you are running ~2200 RPM
You gas mileage will be worse. Your first gear will be just about worthless, and you will probably have to tune it to shift sooner from 1st to 2nd to keep from hitting the rev limiter. That will put you lower in the RPMs when you hit second, below the sweet part of the power band. First fear might be fun with these gears, but it will probably make your car slower, not faster. I would not go above 3.55 on a A6 V6 unless you have something that really bumps up the low-end torque, or you got a custom torque converter to keep you in the power band. In my opinion, it isn't worth the effort until you have built the car to the power you want and you have the right torque converter. When all that's done get the right gear to match your power and output and get the greatest acceleration you can.