The fill-up method: miles since the last fill divided by gallons it took to refill. Get MPG, the metric L/100km equivalent, and cost per mile.
MPG = miles ÷ gallons. The metric figure, litres per 100 km, runs the other way — lower is better — and converts as L/100km = 235.21 ÷ MPG. For an accurate result, fill the tank fully each time and use the trip odometer reading since the previous full fill.
Why is my MPG worse than the sticker figure? Window-sticker ratings are laboratory numbers. Short trips, cold weather, roof racks, and idling all pull real-world mileage down, often by 15–25%.
How do I track a drop in mileage? Log each fill-up. A sudden sustained drop can point to under-inflated tires, a clogged air filter, or a failing sensor.