I thought I lost 7% of my mpg when I filled up with winter fuel which seemed abnormal.
I did some reading and found some citations suggesting "typical" winter gasoline is 10% butane while summer gasoline is only 2% butane. Liquid butane has about 14% lower volumetric energy density than gasoline, so that's only supposed to be a 1% difference unless the butane content is abnormally high or butane disproportionately lowers the density of gasoline.
This chart is completely unrelated to butane but it seems like gasoline's density could possibly vary a lot depending on the blend...that's a 4% difference right there from adding this "hydrocracker naptha"!
So maybe that winter Nevada fuel really is 7% lower energy than summer California fuel. Drats, I thought I was getting a good deal since it was slightly cheaper, guess not.