Trip fuel cost is one multiplication built on a division: gallons needed = distance / fuel economy, then cost = gallons x price per gallon. In metric: liters needed = (distance in km / 100) x consumption in L/100km, then cost = liters x price per liter. The formula is exact — the uncertainty lives entirely in the inputs, especially real-world fuel economy, which runs 10 to 20 percent worse than official ratings once you add air conditioning, roof boxes, city traffic, and a heavy right foot.
Use your car's actual economy, not the brochure figure: track a few tankfuls (miles driven / gallons filled) or trust the trip computer's long-term average. For comparisons, the math exposes the true price of thirst: over 15,000 miles a year at $3.50/gallon, a 25 mpg car burns $2,100 of fuel while a 35 mpg car burns $1,500 — the $600 annual gap is the honest value of choosing efficiency, and it compounds over a car's lifetime.
Three driving habits move the number for free: steady highway speeds (fuel burn rises sharply above 70 mph), proper tire pressure (underinflation by 1 psi costs about 0.2 percent in economy), and shedding dead weight like roof racks when unused. This calculator turns distance, economy, and pump price into the trip's fuel bill — and the return leg is just the same math again.