Morning Gas Pump Myth MPGEnhance

Pump Gas in Morning


            You may have heard the following fuel saving tip before. Pump your gas early in the morning when it is cold outside because you will get more gas into your tank for the same price. The theory behind this is that gas pump meters are volumetric flowmeters. That is, they measure how much fuel you are pumping based on the volume of the fluid passing through the system, not the mass of the fluid.

            This means that they do not account for density changes in the fluid. As a result, if the gas being pumped is colder then you would theoretically get more fuel in the tank for the same price than if it were at a higher temperature.


            While this theory itself is valid as far as temperature changing the density of a fluid, one part does not prove to be accurate. That is the part where the temperature of the gas changes quickly along with the outside air temperature. As a matter of fact gas at the pump is kept in large storage tanks underground. These tanks reach thermal equilibrium with the surrounding temperature of the underground soil. In fact, as you know, the temperature a certain number of feet underground does not correlate daily with the outside air temperature at all. It cannot fluctuate 20 to 30 degrees in one single day as air temperature can. It is a much longer process.

            In fact it can take a while into the spring season for example for the underground temperature to warm up. This is a gradual process. As a result the temperature of the gas underground would remain fairly constant day to day, slowly increasing in the spring and summer and slowly decreasing in the fall and winter. As a result, pumping your gas early in the morning would make no difference to pumping it the day before during the hottest time, noon.

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