Calculate fuel cost for any trip distance.
Trip fuel cost is distance divided by fuel economy to get the volume burnt, then multiplied by the pump price. The order matters for intuition: economy sets how much fuel the journey consumes and price only scales the bill afterwards, which is why a ten percent improvement in mpg saves exactly as much as a ten percent fall in fuel price. The return leg is included by default because most trips are round trips and forgetting it halves the estimate.
Fuel volume
Gallons = miles driven / miles per gallon
Trip cost
Cost = gallons x price per gallon; cost per mile = cost / miles
No. Official figures come from standardised cycles and real-world economy typically runs 10 to 15 percent lower. Divide your own recorded miles by gallons filled over a few tanks for a figure you can trust.
Aerodynamic drag rises with the square of speed, so power demand rises with the cube. Above roughly 55 mph economy falls steadily, which is why the same route can cost noticeably more when driven fast.