Haha, d'ya know, I was thinking the same thing recently, the car's had some gentle runs round the peak district on one tank, and a run to huddersfield and back on another, and has gone from 35mpg tank average to 40-41mpg average,
I'm still having to fill up the same amount each week, so i see it as getting miles for free rather than more miles, but every little helps. It's just nice to know my car isn't broken and it's just the route to work.
On a late night drive from my girlfriends to mine i can get a trip average of low 70's mpg if the lights go in favour.
(ok, the route starts off with a steep downhill descent and is then 40mph dual carriageway on a gentle downhill slope into a brief bit fo 30mph residential but 70's 70 right? )