Miles Per Gallon Calculator

What Is Miles Per Gallon (MPG)?

Miles per gallon (MPG) is the standard U.S. measurement of fuel efficiency — it tells you how many miles your vehicle travels on one gallon of gasoline. The higher the MPG, the less fuel you burn per mile and the less you spend at the pump. The U.S. Environmental Protection Agency (EPA) uses MPG as its official fuel economy metric under the Corporate Average Fuel Economy (CAFE) standards, requiring manufacturers to meet minimum fleet-wide MPG targets.

Outside the United States, the same concept is expressed differently. In most of Europe, Canada, and Australia, fuel efficiency is measured in litres per 100 kilometres (L/100km) — the inverse of MPG, where a lower number means better efficiency. The UK uses miles per Imperial gallon, which produces higher numbers than US MPG because an Imperial gallon is larger (4.546 L vs. 3.785 L). Use our MPG to L/100km converter or UK MPG calculator to switch between standards instantly.

How to Calculate Miles Per Gallon

The miles per gallon formula is simple:

For example: if you drove 350 miles and used 12.5 gallons, your fuel efficiency is 350 ÷ 12.5 = 28 MPG. Our calculator does this math for you — just enter your odometer readings (or total miles) and your fill-up amount.

What Is Good Gas Mileage?

According to the U.S. Department of Energy (DOE) and EPA fuel economy data, the average new car in America achieves around 28–32 MPG combined (city and highway). A "good" MPG depends heavily on vehicle class:

What Affects Miles Per Gallon?

Multiple factors influence real-world MPG, and understanding them helps you improve your fuel efficiency. The EPA's Worldwide Harmonised Light Vehicles Test Procedure (WLTP) — adopted internationally to standardize testing — shows that actual on-road MPG can vary significantly from window-sticker estimates:

• Driving speed: Fuel efficiency typically peaks around 45–55 mph. Highway speeds above 65 mph increase aerodynamic drag exponentially, dropping MPG by up to 15%.
• Acceleration and braking: Aggressive driving — rapid acceleration and hard stops — can lower MPG by 15–30% in city driving, according to DOE research.
• Tire pressure: Under-inflated tires increase rolling resistance. The DOE estimates that proper inflation improves MPG by up to 3%.
• Vehicle load: Every 100 lbs of extra weight reduces fuel economy by approximately 1%, per EPA data.
• Engine condition: A poorly maintained engine — worn spark plugs, dirty air filters, old motor oil — can cut MPG by 4–40%.
• Fuel type: Ethanol-blended fuels (E10, E15) contain less energy per gallon than pure gasoline, reducing MPG by 3–5%. Use our diesel MPG calculator for diesel-specific calculations.
• Climate and terrain: Cold temperatures thicken engine oil and increase warm-up time; hilly terrain demands more engine power. Both reduce MPG.

MPG vs. L/100km — What's the Difference?

MPG and L/100km measure the same thing — fuel efficiency — but from opposite directions. MPG answers "how far can I go on one gallon?" while L/100km answers "how much fuel do I need to travel 100 km?" The conversion formula is:

L/100km = 235.21 ÷ MPG (US)

So 30 MPG equals 7.84 L/100km. Use our MPG to L/100km converter for instant conversion, or the MPG to km/L converter for the metric system's positive-direction format.