As a guide for you, I got 650, nearly made it to 700 miles out of my MK5 2.0 gttdi, I was driving like a granny tho, 55mph tops.

my daily milage is 32 each way, and like you its 80% 2 or 3 lanes.

i may get 450 to 550 out a tank. and i'm normally 50 - 90 mph where conditions and traffic allow.

if you are still on the fuel you got with the car then forget it. fill the tank up yourself, brim full, then you know it is, reset the trip computer and off you go. i find i can get around 100 - 120 miles before the needle has started to move.

ben