Where does the gasoline come from?
https://en.wikipedia.org/wiki/Irving_Oil_Refinery
There's no oil refineries in any of the six new england states, but there is a big Irving Oil refinery, 300,000-bbl/day, in Saint John, New Brunswick, which definitely helps to keep the gasoline and heating oil prices lower for the new england area.
New Jersey has three, Pennsylvania has four, and Delaware has one oil refinery.
https://en.wikipedia.org/wiki/List_of_oil_refineries
So's it could be the lower price in Maine is a result of it being closer to Saint John via the atlantic.
Suggest you look at the list of refineries around the world to get a comparison on how big is 300,000-bbl/day.
"As of January, 2015 there are 137 operating oil refineries in the United States" with none in the six new england states.
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Laconia, NH: 43-31-39 N x 71-28-13 W
Irving Oil refinery - Saint John, New Brunswick: 45-16-44 N x 66-0-45 W