Fuel does expand when hot, however most fuel tanks are underground and maintain a fairly cool an even temp. Also the difference in volume between 60 and 90 degrees is very small (1.7%). I doubt you can buy 90 degee gas many places. On a hundred degree day with the AC off my upstairs might hit 85-90 but the basement is much cooler, perhaps 75. How many above ground storage tanks have you seen? I'd be surprized if underground storage varied in temp by more than 15 degrees year round.