Gas Mileage (MPG)

At its core, this calculator divides the distance you traveled by the amount of fuel your engine consumed during that journey. Enter your trip miles (or kilometers) and gallons (or liters) used, and it returns your gas mileage in miles per gallon (MPG) along with the metric equivalent in liters per 100 kilometers (L/100km). The key to accuracy here is measuring a full fuel cycle: fill your tank completely at the start, drive your normal routes, then fill up again and record exactly how much fuel you added — that second fill-up quantity is your true consumption figure for the distance traveled.

MPG and L/100km express the same underlying idea but from opposite directions. MPG is a 'higher is better' ratio — the more miles you squeeze from each gallon, the more efficient your car. L/100km is an inverse measure — lower numbers mean better efficiency. A car getting 30 MPG is consuming roughly 7.8 L/100km. Understanding both matters because fuel economy comparisons between American and European or Australian vehicle specs are otherwise impossible to make directly. This calculator handles the conversion automatically so you never have to do that math manually.

Several real-world factors cause your measured gas mileage to diverge from the EPA sticker on a new car. Highway driving at steady speeds typically yields 20–35% better fuel economy than stop-and-go city traffic because idling, frequent acceleration, and braking all waste energy. Tire pressure, vehicle load, roof racks, air conditioning use, outside temperature, and even the ethanol content of your fuel (E10 vs. E15) all shift your MPG meaningfully. Tracking your gas mileage over multiple fill-ups — not just one — gives you a reliable baseline and lets you spot trends like a gradual decline that might indicate a clogged air filter, underinflated tires, or a failing oxygen sensor.

A common mistake users make is measuring distance from memory or odometer glances rather than recording the exact reading at each fill-up. Even a 5-mile error on a 300-mile tank can throw off your MPG by nearly 2%. For the most reliable data, reset your trip odometer the moment you leave the gas station after your first fill, then read it at the next fill-up before moving the car. If your vehicle has a built-in fuel economy display, treat it as a helpful estimate rather than ground truth — those systems often read 3–8% optimistic compared to the fill-up method used here.