I received this by email. I've heard that pumping fuel when it's colder makes little difference, but I don't know enough about the properties of gasoline to dispute/debate the rest of it. Just thought I'd share- the link at the end takes you to a page that talks more about the technical aspects of what I'm posting.

TIPS ON PUMPING GAS

I don't know what you guys are paying for gasoline.... but here in California we are paying up to $3.75 to $4.10 per gallon. 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 is diesel the next day is jet fuel, and gasoline, regular and premium grades. We have 34-storage tanks here with a total capacity of 16,800,000 gallons.

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.

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. You should be pumping on low mode, 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 of the 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.

One of the most important tips is to fill up when your gas tank is HALF FULL. 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.

snopes.com: Tips on Pumping Gas

 

i have heard that since the tanks underground are so well insulated, that pumping gas in the morning or during the day doesnt really make a difference.

 

That's what I've heard too.

 

The tip about not filling up when a tanker truck is there is a good one-storage tanks always have some water in the bottom (condensation) and dumping a few thousand gallons in disturbs this off the bottom.

 

They are insulated by the ground around them. The temp in the air does not affect ground temps very much, so this bit about filling up in the morning when the ground is cooler is bull. How do you think geothermic heating and cooling works? It works because the ground temp is more or less constant whereas the air temp is not.

 

I heard and read about this many times. The point he made about fueling up early in the morning and late in the evening is a good idea. Temperature swings in the outside ambient temp aren't as high as they are during midday.

I do understand the fundmentals with insulation and the grounds ability to insulate. Yout gas tank is going to fluctaute with the outside air temp as well. I think this point has been debated about how someone drove so many miles @ 7am and then drove back that afternoon and filled up again with less gas when they refilled. I refuse to fill up during mid-day unless it's an emergency. I fill up my trucks and our car early morning or late in the evening to ensure I get as close as I can with my when calculating my MPG, since I cannot calculate for the expansion of gasoline.

His point with pumping on the low mark is the best idea. Everyone sems to be in such a hurry to pump gas, even when it was $0.77. Here's the Best 2 points to pumping low:
Pumping slower allows the filter(some you can see, some you cannot) to trap more of the mud, water, and other trash that is in the fuel. If you pump @ the High mark, it's still trapping some but if the filter is near it's capacity, it will allow some trash to become dislodged on it's way thru the system.
Pumping slower allows for less vapors to be produced, less sloshing and lower backpressure. Lower backpressure allows for a smoother flow of the product and lowered resistance.

A side note for those who haven't seen work done on pipelines when they repaired. You can only imagine how much mud and water is in the storage tanks. Not mentioning rust either. These same pipelines carry unleaded fuel, diesel, jet fuel, and fertilizer. When this ine becomes damaged, whether from excavation work or someother form, I've seen welders shove dirt, mud, rags, anything they can inside the pipe to prevent the product from coming back into the area they are welding..even when the line has been pumped down.

This isn't an arguement saying that it's bad to fuel up during the day or you shouldn't pump fast. It's more of technical assessment. I learned this 12 years ago when I worked for Williams Pipeline,a company that handled transport of said products. The old foreman I had always said to pump early and we always did since we always checked our fleet trucks over before heading to jobsite in the middle of BFE! It made sense, 2 were gas and 3 were diesel with nurse tanks for our trackhoe, dozer, and backhoe. A lot of diesel on a daily basis.

I think when fuel hit's 5, 6, even perhaps $7.00 a gallon, we may all be looking at how we can maximize every cent of fuel we pay for.

 

Then we are talking about the temp diff between your tank (ambient air) and the tank in the ground. But that is not what the email says. It says fill up in the morning when the ground temperature is still cold. That is pure-dee bunk right there.

The fuel might expand in your tank on a hot day so that volume of the gallon you had in the ground is less than the gallon in your tank. But the diff is so small, I doubt it makes any diff when you are only pumping about 20-30 gallons.

 

starting off slowly rather than a jack rabbit start......just once,will be more important than what time of the day you fuel up.
take off slowly all the time,and you'll really see some gains that make vapors and time of day fueling,of little concern to you.
go one step further now if you can,and let off the fuel as soon as possible to let your truck roll to the stop light/sign rather than braking hard once up there,and you've just done two hypermilling tips that will blow anything else you do as far as when fueling up,totally out of the water.
for those interested learn more here;
http://eartheasy.com/move_fuel_efficient_driving.html
http://ecomodder.com/forum/EM-hyperm...ecodriving.php
http://ecomodder.com/forum/fuel-econ...ifications.php