What RPM to change gears?
 

I know it sound like a silly question, but I always wondered since in A4, our max HP/torque is not in 6K RPM, why we still have to wait until red zone to change gears. I kind of tried changing at lower RPMs, but I could not get more power. Does anyone know what would be the best RPM to change gears in a Quattro 1.8T.

I'd say around 3700rpm for normal driving and 5200 for better acceleration.

You don't have to wait...
 

...nor would you in any car necessarily. You shift at the point that the NEXT gear will be at the engine speed that you want. Sometimes you let the engine run a little past where it needs to in one gear to catch the next one at peak torque or nearer the power peak. The redline is simply where the engine can't run reliably past without blowing up (eventually!).

With the lower power peak and great flat torque "curve" of the 1.8T, you should ignore the redline and shift to maximise it's capabilities.