In Washington state you can't get anything but gas with 10% in it from mid fall to mid spring it is mandated by the state.
You also have to remember, a 10% reduction in mileage @ 47 is far more of a hit than 10% at 15 mpg in other words, if I take a 10% hit for gas & a 10% hit for the cold and a 10% hit for snow & slush on the road I am looking at maybe 34 or 35 mpg....I am running about 36-37 right now so that does not sound too far off.
In my truck at 15 mpg, if I make the same allowances, it comes out to about 11, I am getting about 12 right now, so, well that sounds about right too.....the difference is I am taking a 3mpg hit in the truck & a 10mpg hit in the HCHII yet it works out to about the same % drop.