Just a guess but most oif the gasoline we now purchase is blended with (corn based) ethanol. And to my knowledge ethanol has a lower flash point than oil based gasoline.
The flash point temperature can be used as an indicator of the amount of power received when igniting or burning a hydrocarbon.
The ethynol they add doesn't have the same energy content as gas. That doesn't account for 20% though. More likely those running the "test" are comparing the mileage of the vehicles using winter gas formulations compared agains summer formulations. That'll getcha by 20% every time.
I can run E85 in one of my cars, and even with 85% ethynol the mileage only drops by about 15% when compared to the same seasonal formulation of gasoline.
A over enthusiastic entity must be propogating the Big Oil is bad message by comparing apples to pears (winter formulations against summer).
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