I recently bought a used 2006 VW Jetta because I travel long distances and needed a vehicle that got better fuel milage.  The engine only had 78,000 miles on it, but it ran rough and it always downshifted going uphill.  I was disappointed to find that I was only getting about 35 MPG.  My first thought was that the previous owner had not performed all the necessary maintenance so I took it to the VW dealership and had the transmission fluid and filter changed, changed the fuel filter, and had the tires rotated and balanced.  I changed the oil (did not know that you had VW compliant oil so I used Amsoil), without any benefit.  I intended to use the Lucas Fuel treatment, but when I mentioned that to the service tech at the dealership, he said that it was not a good idea, because he had seen engines damaged by using it.  I found this hard to believe because I have used in all my other vehicles with good success.  However, based on that recommendation, I waited.  After 3,000 miles and no improvement, I stopped at an autoparts store and bought a quart of fuel additive, and put the whole thing in the tank at the fill-up.  I drove about ten miles and started to notice some smoke from the exhaust, and I became concerned that maybe the service tech had been right about engine damage.  But then the smoke diminished, but was still evident, and the engine started to run smoother.  100 miles later, no smoke, the engine runs smoothly, it no longer downshifts going up even steep grades.  I now get at least 42 MPG and the car is wonderful. 

Now I add some at each fill-up.  Thank you for a simple fix.

Randall Barron