Because gasoline expands and contracts based on temperature, the industry uses a standard temperature 60 degrees Fahrenheit -- as part of its definition of a gallon. To the industry, a gallon of gasoline is the amount of fuel that occupies 231 cubic inches at 60 degrees.
But at 75 degrees, the same amount of fuel occupies 233.4 cubic inches. At 90 degrees, the gas expands to 235.8 cubic inches.

Meaning you will get less fuel into your car tank when fuel being pumped has already expanded.

Its always advised to avoid fuelling your car When:

1. Its extremely hot. (Fuel when its cold, early in the morning)
2. The Gas stations' underground tanks are being re-filled or have just been re-filled.

Variations will be more noticable when filling a full tank.

A high number of Petro companies/stations will not sell super/premium if ur buying using jericans.

[quote=MaichBlack]
I'll give you an analogy of a mountain bike. When you change the gears, and subsequently start cycling, the distance covered will be different [more or less] per cycle. Fuel pumps use the same concept and adjusting the size of the those circular things that are rotated [can't remember the name] gives erroneous readings. You can achieve higher readings [or lower if you wanted] for the same amount of fuel.

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@Maich,
I almost fell of my stool with laughter....well,i suggest you present your physics teacher to me for hanging