In every mileage thread, I see people blaming "Winter Gas" for part of the mileage drop during the season. Technically speaking, winter gas has almost the same energy content, it is roughly 2.5% less energy, (we'll leave the math out, but that's pretty close) at most, if more Ethanol is added. Fuels where I live have ethanol all year,and the proportions do not change much at all seasonally, it's more of an availability issue.. Winter fuel is generally formulated to vaporize easier, allowing easier starts, and combustion. That is why summer fuel is different, to prevent vapor lock. A 2.5% loss in energy, may affect fuel mileage at most 2%, which is generally not measurable.
However, as read on a ScangaugeII, a warm start idle will burn around 1.25 LPH (liters per hour). A cold start at 5C will start off around 5 LPH, and for the first 10 miles or so, mileage is poor, before the mixture leans back out, fluids heat up, and my mileage goes to it's normal highway mileage.
So if starting cold at 5C uses 4 times the fuel, -20 or colder start must use closer to 10+ LPH at idle, and takes around 20 miles to go to it's winter normal mpg number. So just the act of starting it, never mind idling, uses lots of extra fuel. Denser air, emissions considerations, you name it, it's working against fuel economy. Shorter days using lights more, blower fan cranked, and on and on//
It's not the gasolines fault.