Its an old racing car trick...........yes the cooler the gas the more dense it is..........pump up when it is coolest (usually between 4 to
6am)......
race cars used to put a dry ice cooler hidden in cars to run gas lines through to create a more dense gas mixture to increase horse power.......
but in your case the gas will "heat" up and expand in you tank as the day warms up and gives you some slight fuel advantage......

