I've only had my tdi 170 Quattro for 5 weeks. My weekly commute is 300 miles, 30 miles each trip, a mix of a and b roads. The first 2 weeks I was getting an average of 34mpg. I've got a problem with excessive smoke/rough idle/hesitation so I used a can of BG244 fuel additive to flush the engine out. Since using that I've been getting an average of 42mpg. Identical journeys and driving style. The problems are still there but the fuel additive was well worth £24 a can! Tyre pressures can also make a big difference.