I have been experiencing pretty weird (mildly speaking) issue with gas mileage for a while. I have Compass Rallye 2007, 2.0 CRD (Volkswagen diesel engine). I know that you are not so familiar with engine, but I hope you can give me some advice.
During my first year with Compass (bought it in April 2011) I used to have average city fuel consumption of 10-11 l/100km (21.3 - 23.5 mpg), except for very hot summer days, when it went up to 12.5-13 l/100km (18.5 mpg) because of mass AC usage. But, in last year (June 2012 - ) I managed only 13.5-14.5 l/100km (16.2-17.4 mpg) during regular days and it`s deteriorating, to above 15 l/100 km (15.7 mpg) in summer. I think that these results are pretty bad, considering that this very same engine in VW Passat/Golf or Audi can return city average of 10 l/100km (23.5 mpg). The results in highway have not changed much, but have changed. They used to be 6.5-7.0 l/100km (33.6-36.1 mpg), and now they are 7-8 l/100km (29.4-33.6 mpg). What can be causing this?
I am not a long distace crosser, drive only about 10-15 K km in a year (6200-9300 miles), most of it in a city. Also, this car produced a lot of mechanical and, especially, electrical issues during these 2 years. Exactly a year ago, I changed TIPM (500€=651$) plus some thing, in my domestic language, called "electric installations hub" or "electric installations relay" (located behind left front wheel) which also cost 500€, but I bought used one for 10€. This may be a misleading clue for you, but I had to mention it. Other mechanical issues were not related to engine (struts, tie rods, silent blocks,...).
Thanks for reading, hope for some help.