I do fuel purchase optimization manually because I don't have Promiles. It's a little time-consuming, but I have all of the fuel chains bookmarked on my toolbar....
Ever thought about using a Garmin? One of the functions is Fuel and it points you in the direction and gives you last known price (usually today's).. then you can head for the cheapest, the furthest out and really maximize your savings.
Additional... it creates a list at the touch of a button...