I use a nifty little program called Fuelly. www.fuelly.com.
I never partially fill the tank and have done 4 fill ups and 1113 miles.
All of my mileage is about 65% freeway and these number relect that. My best is 24.7 and avg. is 23.7
I am always on cruise on the freeway at approx 72mph, climate control on at 72 deg.
It tracks fuel prices, avg. price, cost/mile and other little things.
Good news is that it's free.