[Big Dave]

I live in Indiana and I don’t think they sell much in the way of winter blend here. I’ve had tanks gel up (“cloud” is the ASTM term) at 7 below here. Maybe at big truck stops up along the Indiana Toll road that serve trucks that may go farther north, but few other places here.

As for the heat content of winter blend, it is proportional to the No. 1 content. The heat value of straight No. 2 is 139,000 BTU/gal. For No. 1 the heat content is 130,000 BTU/gal. So if you winter blend were the outstanding stuff you get in Nebraska and the Dakotas (66% No.1) you get a heating value of 135,900 Btu/gal or about 2% lower heating value.

Most stations that sell real winter blend are quite proud of it (it sells at a considerable premium) so if you have to ask, it probably isn’t winter blend.

Now the decrease in MPG is very real, but I’d ascribe the reduction mostly to increased aero drag.

Remember the part of the road load equation that deals with aero drag.

HPaero = kρCdAV3 where
K is the consolidated conversion constant
Ρ is air density
Cd is coefficient of drag
A is frontal area
V is velocity
From standard psychrometric tables we see that:
At 0°F ρ=603.7 grains/ft3
At 70°F ρ=520.0 grains/ft3

We see that when air temperature drops from 70 to 0 the air density increases by 16.1% and thus the biggest component of road load goes up by the same amount.

Winter blend MPG loss = 2%
Denser air MPG loss = 16%

Winter has other effects that I cannot quantify.
a. Increased rolling resistance from snowy/slushy road surfaces
b. Driving conditions (including other drivers) make hypermiling difficult