MPG - am I imagining it?

[Smarty]

Just a quick question, from all you experienced and clever lot. (I should say I have a Mk6 280 SWB 1900cc diesel)
I got my van Oct 05, with 37,000 on the clock, and I know I am getting old but the mpg, I am sure was good, and I am sure I was getting 600 miles plus to a tank.
Now I have just crossed 50,000 miles and am getting about 500 plus to the tank.
I could just be getting old, and mixed up.
Could this be right, if so is it something I have done, or have not done?

Sorry if I am wasting your time.

Smarty

[sc_BK]

Depends how low you're running it down aswell. I usually get 450-500 miles before filling up with about 60l - the mk6's have 80l tanks