I've a 2007 audi a3 around 150k miles 

in my first year of having the car at 135k miles driving to my college I was getting mid 30 mpg in the city and on the high way I was high 30 and sometimes in the 40s. It's now year 3 of having the car and I started to notice on year two that it was starting to decline. I've had my spark plugs changed from iridium to platinum and my waste gate for the turbo change (no retuining done) I was also using regular gas instead of premium due to it being so expensive especially while at  college. I've now switched to premium on. I've used some fuel additives such as seafoam which has brought it up but not where it was in the beginning. I had a mechanic who use to work for vw check the fuel filter and fuel injectors which came up working fine. 

regards nick w.