Just bought a 2005 Beetle TDI 100,500mi. First check of mileage was 30mpg. Using A/C most of time. Got into independent shop changed out fuel filter. Shop says no intake blockage when fuel filter was changed. Just drove 150mi, still 30mpg. We will have timing belt changed at end of month since never changed and will get tire pres up, but what else could be causing the low mpg?