2005 TDi - drop in fuel mileage

Hello from the USA,

I have experienced a drop in mileage from an overall average of 41 mpg to approx. 36 mpg.

This drop in mileage has been consistent over the last four tank fills and commenced after my 2005 TDI reached the 51000 mileage mark. My driving habits have not changed (60% city / 40% motorway, Max. speed 75 mph).

I changed the fuel filter at 47000 miles. I had changed the airfilter twice within that period as well. I change the OEM-specified oil and filter every 7,500 miles.

I consistently purchase the diesel from Shell - though not always from the same petrol station. The cetane rating is either 42 or 45, depending on the station.

Any thought/suggestion on the matter would be appreciated.

Thank you.