The well-insulated tanks in the ground will tend to keep the fuel at the temperature it was delivered from the tanker truck, not the temperature of the ground. The fuel is not in the underground tanks long enough to benefit much from ground temperature.
Some reports say filling up in Phoenix could short you a quarter gallon per tank over the same fillup in Minnesota. Sure it's trivial if you only consider one tank or one person, but consider the millions of tanks and 125+ billion gallons sold last year in the US.
Now if I could only get every person in the US to send me a measely dime....
