Viewing my past results on Fuelly, the most I've ever filled up with was in July 2014; 14.638 gallons after traveling 505 miles. City type driving was estimated at 40%. Got 34.5 mpg. Previous best tank (before breaking the 40mpg barrier) was in Sept 2014 on a trip to Lava Butte and back. Drove 433.2 miles and filled with 11.024 gallons and achieved 39.3 mpg with only about 10% city type driving.
One number I started tracking recently (since May 7, 2015) is average mph for the tank. I've found that under the right conditions, with little or no city type driving, mid to high 30's mph is best for achieving great mpg. Anything below or above that, I start to get diminishing results.
"The secrecy of my job prevents me from knowing what I'm doing."