I do see your point about driving slower will increase your mpg; however, I have to agree with others. The 8.9 sounds really good, but in reality it means nothing as too many factors will come to play. Only measuring the "average" mpg over a long period of time, lots of miles, and different driving conditions can one truly know his mpg.
I have a small excel work sheet that I use to keep track of miles, gallons, price, and mpg at each fill up. With my previous dp, over a period of about six years, I averaged about 8.6 over 36,000 miles...with single measurements varying from 5 to 12 mpg. Our new dp, with only 4000 miles on it I'm only averaging 7.4 mpg.
Continue to keep track and only be concerned with the average mpg.
Happy travels..
Ron