Poor quality gasoline seems to be a prevalent issue these days. When my wife and I travel, the fuel economy in the same vehicle, same load, same speeds, nominally the same octane gas, we routinely have fuel efficiency variations that sometimes go as high as 20% or more after using up the fuel and filling the tank elsewhere. I am writing about late model vehicles with the mpg display on the instrument cluster but the miles driven and the instrument cluster mpg check out so it is not some calibration issue and it happens on virtually al of our trips. Yes, there are different formulations for fuel when travelling long distances, but it is difficult to see how anyone benefits from a different fuel formulation if it knocks 20% or more off of the fuel economy. Burning 20% or more more fuel would seem to more than offset any claimed benefit to the different formulation in terms of emissions.