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Fuel Saving Tips

The Myth about Filling Up in the Morning

A popular tip advising the best time to fill up your car's fuel tank comes to us as an unfounded urban "myth." The "tip" advises motorists to fill up the tank first thing in the morning or a few hours after the sun goes down. The idea behind the myth is that when you tank up during the cooler hours, the fuel would be denser, and thus you will be able to fit more fuel into your tank than you would by tanking up during warmer hours. Because most fuel station pumps measure fuel by volume, theoretically you would actually be getting more fuel molecules into your tank during the cool hours, too. The idea is that you would get more fuel for the money you pay. Nice idea, but this is an unfounded proposition. Here's why: Fuel is stored in large tanks located so far underground that they are unaffected by warmth from the sun and outside air temperature. Being this well insulated by the earth, fuel in these tanks remain at the same temperature all day long, without any regard to the sun. Thus no matter what time of day you fuel up, you will be pumping fuel that remains the same temperature throughout the day. The only factors that may affect underground tank temperatures are: 1) when a tanker truck introduces new fuel into the underground tank, and 2) during seasonal temperature changes, from summer to winter, when the underground temperature may vary by a few degrees. The good news is that underground tank temperatures are relatively stable, and you can pump fuel into your car confidently knowing you are getting pretty much the same amount of fuel per-gallon (per-litre) no matter what the temperature is outside. Tank up, and drive off!

posted by PriusCMPG on July 17, 2012

this tip works for 87% of voting Fuelly members.


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