A lower price only saves money if it's more than the fuel you burn getting there.
How this works
Extra fuel cost is the extra miles divided by your mpg, multiplied by 4.546 litres a gallon and the cheaper station's price. It leaves out wear on your car and your time.
How the results page estimates this
It compares the cheapest station with the nearest open one, there and back. Distances are straight-line, so we add 25% for roads, and use 40 litres. The mpg and road figures are rough estimates, not official figures, so your own numbers will be more accurate.