Chris
I've had this happen several times. It seems to happen to me when the fuel foams very little

That's normally a indication of a quality fuel. It's one of the things that is measured in European fuel standards. I've been buying most of my fuel from Meijer and it's labled "permium diesel". I've used it in my diesels for over 15 years or whenever they came to Columbus with good results. Also their stores are located in competive areas and usally it's about the lowest priced around

But that's a realative term
On these overflowed tanks as I read your notes is when you got your best mileage. Part of your increase could be the fuel. On my last jetta tdi (99.5 A4 platform) I actually got two tanks in the low 70's

I attribute most of it to fuel from Florida and Georgia. My experience is the further south you go the better the fuel at least on the eastern part of the country. The fuel we get in Centeral Ohio seems to be the worst quality on the plannet

Meijer fuel is supposably trucked in from Kentucky
