Compare fuel prices across stations.
Driving further for cheaper fuel is worth it far less often than people assume, because the saving is capped by the size of your tank while the cost grows with every mile. A 16 cent per gallon gap on a 12 gallon fill saves 1.92 dollars — and at 28 mpg, each extra mile costs about 12 cents, so the break-even detour is 16 miles round trip. Anything beyond that and you have paid for the privilege. The break-even distance is the number to remember: it scales with the price gap and the fill size, so it is generous on a big tank and almost nothing on a top-up. This ignores the value of the time spent, which usually makes the case weaker still.
Fuel Price Comparison
Gross saving = price gap x fill volume; detour cost = extra miles / mpg x fuel price; net = gross - detour; break-even miles = gross saving / (price / mpg)
Divide the total saving on the fill by your fuel cost per mile. For a 20 cent gap on a 12 gallon fill at 28 mpg, that is about 20 miles round trip — but the saving is only 2.40, so your time matters more than the fuel.
Yes, proportionally. The saving scales with gallons bought, so a 25 gallon truck tank justifies twice the detour of a 12 gallon car for the same price gap.