If you stay with in 3500rpm or 70 mph cruise, you'll get the 20+mpg. If you keep on flooring and getting on it all the time. Your tank will probably last 10 miles per tank.
Like on a race track, I was getting only 5 at the most 1/4 mile passes on 10 gals of tank. That is 1.25 miles per 10 gallons of gas.