I have made no modifications. I fill my tank to it's fullest capacity and then drive. Here are my other results:
723 miles 18.6 gallons 38.9 mpg
743 miles 18.2 gallons 40.8 mpg
719 miles 18.0 gallons 39.9 mpg
All of these results have been achieved during the spring/summer months. I have found that I can easily go another 75 miles after the --- comes up on the screen. However, I am a little nervous of running out of gas. Here are the mileage amounts I am getting according to the gas gauge:
250 miles with 3/4 tank left; 450 miles miles with 1/2 tank left; 625 miles with 1/4 tank left.