[magnets]

SoftwareBug is right. Rotation won't be dependent on the gyro angle if you are specifying speed. However, if you would like to tell the robot which angle to be at (ie, point forward, point 135 degrees to the right) you need a PID loop, but I don't know of any team that has used this for their normal driving mode.

If you want to control mecanum in the traditional way with field-centric control, you need to convert the x and y desired speed to polar, add the gryo angle to theta, and convert back to x y. All of this is taken care of in the LabView, Java, and C++ libraries from FIRST. If you want robot centric control, just put 0 as the gyro angle, otherwise set the gyro angle from the output of your gyro.