Aerodynamic drag rises proportionally to the square of your velocity, but gearing and the engine's power curve are also significant factors, complicating any kind of simple answer as to what is the "best" speed for mileage/time elapsed. You are probably getting less MPG at 95MPH than 65MPH (maybe 1 or 2MPG less?), but the difference between 65MPH and, say, 80MPH is probably negligible.