Going out on a limb here. I have not personally verified this information. It was given to me via email and seems credible enough. Wanted to get it out here for some input from others.

TIPS ON PUMPING GAS - FROM EMAIL SENDER

My line of work is in petroleum for about 31 years now, so here are some tricks to get more of your money's worth for every gallon..

Here at the Kinder Morgan Pipeline where I work in San Jose , CA we deliver about 4 million gallons in a 24-hour period thru the pipeline. One day it's diesel the next day it's jet fuel, and gasoline, regular and premium grades. We have 34-storage tanks here with a total capacity of 16,800,000 gallons.

PUMP IN THE MORNING

Only buy or fill up your car or truck in the early morning when the ground temperature is still cold. Remember that all service stations have their storage tanks buried below ground. The colder the ground the more dense the gasoline, when it gets warmer gasoline expands, so buying in the afternoon or in the evening....your gallon is not exactly a gallon. In the petroleum business, the specific gravity and the temperature of the gasoline, diesel and jet fuel, ethanol and other petroleum products plays an important role.

FILL TANK AT LOWER PACE

A 1-degree rise in temperature is a big deal for this business. But the service stations do not have temperature compensation at the pumps. When you're filling up do not squeeze the trigger of the nozzle to a fast mode. If you look you will see that the trigger has three (3)stages: low, middle, and high. In slow mode you should be pumping on low speed, thereby minimizing the vapors that are created while you are pumping. All hoses at the pump have a vapor return. If you are pumping on the fast rate, some other liquid that goes to your tank becomes vapor. Those vapors are being sucked up and back into the underground storage tank so you're getting less worth for your money.

FILL TANK WHEN HALF FULL/EMPTY

One of the most important tips is to fill up when your gas tank is HALF FULL or HALF EMPTY. The reason for this is, the more gas you have in your tank the less air occupying its empty space. Gasoline evaporates faster than you can imagine. Gasoline storage tanks have an internal floating roof. This roof serves as zero clearance between the gas and the atmosphere, so it minimizes the evaporation. Unlike service stations, here where I work, every truck that we load is temperature compensated so that every gallon is actually the exact amount.


Another reminder, if there is a gasoline truck pumping into the storage tanks when you stop to buy gas, DO NOT fill up--most likely the gasoline is being stirred up as the gas is being delivered, and you might pick up some of the dirt that normally settles on the bottom. Hope this will help you get the most value for your money.

Snopes to the rescue
snopes.com: Tips on Pumping Gas

With most of the above the differences are so minuscule that you will never notice.