Bought a 2009 VW Jetta TDI in April and have yet to have a tank of diesel that will cause my average MPG curve to have a negative slope. Coming from an 1986 Mercedes 300 SDL who would soot at ever pedal stomp, I cannot find a way to make an exhaust soot cloud in the TDI, even under 0-60 race conditions. Diesel prices have been less than mid/premium unleaded in California since I bought the car.