Does pumping gas in the morning, pumping gas more slowly to avoid vapors and refilling more often save money?

Question

There is currently an article circulating on the internet that claims to give tips on saving when filling up on gas. The full article can be found here:
http://www.snopes.com/inboxer/household/gastips.asp

I was hoping someone with a scientific background or someone who works in the industry could answer this question. I am curious to know if this is factual or just another urban myth.

Answer 1

The three specific ones you ask about are all true, BUT.

Slower pumping and leaving less empty space in your tank both reduce the loss to evaporation, but I doubt that the difference is enough to notice.

However, pumping in the morning is a particularly good idea if you can swing it. Gasoline has a much higher coefficient of thermal expansion than water, which means that a gallon of gas at 0°C weighs more than a gallon at 20°C, since it has expanded with heat and gotten less dense. Underground storage tanks are fairly thermally insulated, so you really get more of an effect seasonally rather than across the day, but it's there. The gas currently sitting in the hose is fully susceptible to this effect, so ironically if you only fill up a little (to avoid vapors), you get a bigger effect from temperature in the afternoon to offset the savings. OTOH, it also means that pumping just a little at a time and only in the morning probably helps more.

Answer 2

Yes, they are true, BUT not the whole story.

The top of the station's tanks are at least 2-3 feet below ground. Temperature variations there are extremely slow. The fuel temperature in the tank is determined much more by its temperature when it was put into the tank, and how long it has been there. If the pump has been sitting in the sun for hours, that could make a difference.

When you pump a gallon of gas into your tank, a gallon of air saturated with gasoline vapor leaves your tank. It doesn't matter how long it has been in your tank. If your state requires vapor recovery, then those vapors are captured. If not, they enter the atmosphere and become air pollution from unburned hydrocarbons.

If you 'top off' your tank, some of that extra gas is diverted into a separate canister which helps prevent spills from overfilling. From there, it evaporates into the atmosphere, again adding to air pollution but not powering your car.

If you fill up your car with cold gasoline and immediately park your car in the sun, the gas in your tank expands significantly. Guess where the overflow goes.

Modern streamlined automobiles achieve their highest fuel efficiency when driving steadily around 50-60MPH.

Modern fuel-injected engines are efficient at all levels of power output. The old idea of driving like you had an eggshell between your foot and the accelerator doesn't make much difference nowadays.

If you see the light turn red ahead of you but keep your foot on the gas anyway, all that fuel is wasted, since you'll need to stop at the light anyway.

How far out of your way do you drive to get gas, or to get cheaper gas?

Answer 3

For about every 100 lbs you carry in your car, you reduce your mpg by about 2 percent.
lowerhttps://www.autohausaz.com/html/improve_fuel_efficiency.html

I am no gas scientist but know enough about fluid dynamics - all the info at your link sounds reasonable. Especially the heat - gasoline is very volatile and expands, I know that from living in AZ. Pumping when it is colder makes perfect sense.

One problem I have with filling up 1/2 full is that if you have to drive somewhat out of your way to get gas you loose gas driving there. Also hauling a full tank of gas (how much depends on your tank size) reduces mileage somewhat. You also loose all the evaporated gas in the tank during fill-up; less evaporation if you fill up during colder time of day.
I know you should not fill up past the auto cut off to leave evaporation room in the tank. Else you loose the excess gasoline evaporation in emissions

I would follow the advise except consider the refilling. Here in Arizona it is hot in the sun all year around, but if you cooler climate you can certainly take advantage of these helpful hints.

Keeping RPMs below 2500 at all times and driving below 60 I get 37 mpg instead of 30 mpg.