I have a 02 jetta tdi and a 04 passat tdi....I want to increase fuel mileage if possible. It was suggested that I look into just changing the diameter of the exhaust pipe from the turbo to the muffler to a 3 inch pipe. Anyinfo would help

 

3in pipe would open up a good airway and would breathe better, but not sure if it would improve your gas mileage.