Fuel economy getting worse?

[z2g]

I have a 2010 3.0i with about 12k miles on it now. I've been noticing in the past few months that my fuel economy is getting worse. My driving habits hasn't changed.

In the first year, I was getting 19-20 mpg consistently. Now, it's consistently 15-17 mpg. On long road trips, I'll get 20 mpg.

Is that normal? Also, due to my use, I can't bring the car in for maintenance until the two year mark. Could that be it?

By the way, I've always wonder how good it can be on a new car if you don't change the oil and filters for two whole years!?!?!