Of course, some folks have it easy...
Where I live in South Wales, average supermarket gas station prices are £1.329 a litre - that's 0.264172 US gallon.
So 1 US gallon is (1/0.264172 x £1.329 =) £5.031, which at to-day's exchange rate of $1.51 to the £ makes 1 US gallon of gas $7.60.
Cheapest gas currently in Webster, NY (which is where I live when I'm in the US) is $3.72, though last year it was over $4.