MPG Calculator
Calculate your car's miles per gallon from miles driven and gallons used.
The Formula
L/100km = 235.21 / MPG
km/L = MPG × 0.4251
Miles driven = 387 · Used 12.4 gallons
MPG = 387 / 12.4
= 31.2 MPG
How to Calculate Your Real-World MPG
The most accurate way to calculate your car's fuel economy is the fill-to-fill method: fill the tank completely and note the odometer reading. Drive normally. When you fill up again, note the gallons pumped and the new odometer reading. Divide miles driven by gallons pumped to get your MPG. For example, if you drove 312 miles and pumped 10.8 gallons: 312 / 10.8 = 28.9 MPG. For the most accurate result, calculate over multiple fill-ups rather than just one, since a single tank can be skewed by different driving conditions.
Why Your Actual MPG Differs from the Sticker
EPA fuel economy ratings are measured under controlled conditions that may not reflect your driving habits. Real-world MPG is typically 10–20% lower than EPA estimates for several reasons: city driving with frequent stops and starts burns more fuel than smooth highway driving. Highway driving above 75 mph is significantly less efficient than the 55–60 mph used in EPA testing. Air conditioning reduces fuel economy by up to 25% in city driving. Cold weather (below 20°F) can reduce fuel economy by 12–22% for conventional vehicles and up to 40% for EVs.
MPG vs. L/100km: Two Ways to Measure Fuel Economy
The US uses MPG (higher is better). Most of the world uses L/100km — liters of fuel needed to travel 100 kilometers (lower is better). To convert MPG to L/100km, divide 235.21 by the MPG figure. A 30 MPG car uses 235.21 / 30 = 7.84 L/100km. This calculator displays both so you can use whichever system you prefer, which is helpful when comparing vehicles from different markets.
Frequently Asked Questions
Why is my actual MPG lower than the EPA estimate?
EPA testing is done under controlled lab conditions that favor efficiency: moderate speeds, no extreme weather, standardized routes. Real-world driving introduces variables that reduce efficiency: higher freeway speeds, stop-and-go city traffic, cold weather, AC use, roof racks, and extra cargo weight. The EPA introduced updated testing procedures in 2008 to better reflect real-world conditions, but even the new estimates differ from actual results for many drivers.
How do I convert MPG to L/100km?
Divide 235.21 by your MPG figure. So 30 MPG equals 235.21 / 30 = 7.84 L/100km. Going the other direction, divide 235.21 by the L/100km figure to get MPG. So 8 L/100km equals 235.21 / 8 = 29.4 MPG. This conversion is useful when comparing a US vehicle to a European one, or when reading international automotive reviews.
What factors affect MPG the most?
Speed is the biggest factor on highways — fuel economy drops sharply above 55 mph due to aerodynamic drag increasing with the square of speed. Driving style matters enormously: smooth acceleration and anticipatory braking vs. aggressive stop-and-go can differ by 30% or more. Vehicle weight, tire condition, and maintenance state (air filter, spark plugs) also have significant effects. AC compressor load reduces efficiency, especially at low speeds and in hot weather.
Is it worth tracking my MPG?
Yes, for several reasons. A sudden drop in fuel economy — more than 5–10% below your usual average — can indicate a maintenance issue like a clogged air filter, failing oxygen sensor, or low tire pressure. Tracking MPG over time also makes the impact of driving habit changes tangible. Many people drive more efficiently once they can see the immediate numerical feedback.
How does EV efficiency compare to gas cars in MPG?
Electric vehicles use MPGe (miles per gallon equivalent) as their efficiency metric. One gallon of gasoline contains 33.7 kWh of energy, so an EV that travels 100 miles on 33.7 kWh has an MPGe of 100. Most EVs rate between 90 and 130 MPGe, making them 3–4x more energy-efficient than comparable gasoline vehicles. The cost comparison depends on local electricity vs. gas prices, but electricity is typically cheaper per mile than gasoline in most US markets.