similar to the site b3nn3tt posted, I've been using
Portland Gas Prices - Find Cheap Gas Prices in Oregon (or insert your own city) since mid 2005. Since then, my average is 19-24 for everyday driving (rush hour 25 mile each way in stop/go freeway traffic rarely getting over 30 mph) and 26-28 on out of state trips (likely cruising around 75mph with some random stops).
Driving out to CF, I am "planning" on averaging around 28-30 - so as long as I don't get board and speed, think I should come close.
I also have been noting oil changes, sparkplugs and seafoam and have noticed zero difference for the tanks of gas following. Not to say new plugs and oil changes and whatnot don't improve efficiencies, but based on my records, there has not been any noticeable changes for the better - could have just been the tanks following those things had unusually slow traffic or other reasons to bring the mileage down - idk.
(for example, my tank prior to using seafoam averaged 21.3mpg. the one after was 19.29 and the next 19.08. So 550 miles after using seafoam, my mileage lost more than 2mpg.