Gas Mileage Calculator

Miles per gallon measures the car's own fuel efficiency -- how much distance it gets from a fixed amount of fuel -- which depends only on Distance Driven and Gallons Used, not on what that fuel cost. Gas Price only enters the calculation once you move to dollar figures like Cost per Mile and Trip Fuel Cost; it has zero effect on the MPG number itself.

Yes, holding distance fixed -- MPG is distance divided by gallons used, so for the same trip distance, using more gallons directly lowers the calculated MPG. This is the inverse relationship at the core of the calculator: burning more fuel to cover the same ground is definitionally worse mileage, whatever the underlying cause (traffic, terrain, driving style, or a less efficient vehicle).

They point the same direction in terms of efficiency but read oppositely: MPG is "higher is better" (more miles per gallon), while L/100km is "lower is better" (fewer liters needed to cover 100 kilometers). A fuel-efficient car will show a HIGH MPG figure and a LOW L/100km figure simultaneously -- both describe the same underlying efficiency, just inverted.

Directly and proportionally -- Annual Fuel Cost is calculated from your Annual Driving mileage divided by MPG, then multiplied by Gas Price, so driving more miles per year raises the projected annual cost in direct proportion, holding fuel economy and gas price fixed. Doubling annual mileage roughly doubles the projected annual and monthly fuel cost figures.