cruisin on the highway?

We were debating if there would be any gain (more specifically in MPG) by adjusting the spark time for cruisin on the interstate. We're about to leave for a road trip to northern Georgia. So 8 hours of highway driving... we'll say 70-80 mph for the majority of the trip.
Would advancing it help any? What would the risk level be?
all of this assuming we are very light on the throttle getting on and off the interstate when we stop for gas

Just lookin for everyone's 2 cents...