Seasonal fuel economy

Something I just figured out this morning reading a sticker on the gas pump.  I always wondered why I seemed to get better miles per gallon in the winter than in the summer.  The simple answer is that the liquid is more dense at cooler temperatures.  This was confirmed by a sticker on the pump that said “We sell gas by the gallon.  The energy content of the gas is affected by temperature.”  OK then, mystery solved.


2 thoughts on “Seasonal fuel economy

  1. In the U.S. there are winter and summer blends of pump gas as noted here:

    The difference between summer and winter blend gasoline involves the Reid Vapor Pressure (RVP). RVP is a measure of how easily the fuel evaporates at a given temperature. The more volatile a gasoline (higher RVP), the easier it evaporates.

    Winter-blend fuel has a higher RVP because the fuel must be able to evaporate at low temperatures for the engine to operate properly. This is especially true when the engine is cold. If the RVP is too low on a frigid day, the vehicle will be hard to start and once started, will run rough.

    During the summer heat, the RVP of gas has to be especially low to keep it from boiling off. Different states and cities have their own rules based upon their seasonal temperatures. North Dakota for example would have different needs for summer gas than Texas would. One general rule is the lower the RVP of a gas blend, the more it costs.


    Winter fuel is more volatile therefore more energy per unit. Also the air temperatures are lower and therefore the air is more dense, which means there is more oxygen per volume. I believe most automobile motors operate best in 50-65 degree fahrenheit range as sort of an average compromise across the U.S.

    Fuel as gas stations are stored in underground tanks, so I doubt there is much appreciable change in that temperature relative volume.


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