I strongly suspect that prevailing wind has more to do with your mileage than any difference in gasoline quality. Also, if you picked up much road dirt, that'll account for at least a fractional mpg if not a full mpg compared to a freshly waxed car, all else being equal.

BTW- Maine still puts MTBE into their gas. I suspect there may be some performance difference between that and non MTBE. In any case there is a huge environmental difference. The MTBE ends up in our aquifers, even though it was meant to oxygenate gas and reduce emissions. A good idea gone bad. Maine is supposed to stop adding it in Spring I believe.

claude