I've had this suspicion now for some time that the MPG for my Jetta have not been at the level that they used to be. I've posted a couple threads to TDIClub, but the answers have been either vague, suggested replacing the boost lines (which I've already done), or "don't worry about it MPG always goes down in winter". I've always posted summary data to try to avoid the third comments, but it's the Internet.

Anyway, I finally linked to my spreadsheet data and ran some SQL queries against it to generate aggregate data. My pretty picture is below and it clearly demonstrates that average MPG has declined every year from 2002 through 2005. Comparisons are between months across years and so the data controls for the "winterized diesel" effect.

qryAverageMpg (calculates average for each month, groups by Year, Month): 


  SELECT Format([Date],"yyyy") AS [Year], Format([Date],"MM") AS [Month], 

  Avg(Mileage.MPG) AS AvgMpg

  FROM Mileage

  WHERE (((Mileage.Date) Between #1/1/2000# And #12/31/2005#))

  GROUP BY Format([Date],"yyyy"), Format([Date],"MM")

  ORDER BY Format([Date],"yyyy"), Format([Date],"MM");
qryAverageMpg_Crosstab (rotates the data from above so each year's averages appear in a single column): 

  TRANSFORM Avg(qryAverageMpg.AvgMpg) AS AvgOfAvgMpg

  SELECT qryAverageMpg.Month, Avg(qryAverageMpg.AvgMpg) AS [Average Of AvgMpg]

  FROM qryAverageMpg

  GROUP BY qryAverageMpg.Month

  PIVOT qryAverageMpg.Year;

The graph shows that 2001 had the highest data point and that 2002 had the most values consistenly above 50. After 2002, average MPGs for each month drop off.