Article — MPG Calculator
MPG calculator: miles per gallon, L/100km, and annual fuel cost
Miles per gallon equals the distance driven divided by the fuel used. A car that covers 300 miles on 12 gallons is doing 25 MPG, which is 9.41 L/100km or 10.6 km/L. The 2024 EPA fleet average for new US light-duty vehicles was 28.3 MPG (real-world adjusted), the highest figure on record. UK MPG is always about 20% higher than US MPG for the same car because the Imperial gallon (4.546 L) is bigger than the US gallon (3.785 L).
Enter the miles driven, the gallons used, the price per gallon, and your annual mileage. The calculator returns MPG (with an efficiency badge), the metric equivalents, and the cost per mile and per year. Flip the toggle to switch between US and Imperial gallons.
What MPG measures
MPG (miles per gallon) is the standard US measurement of vehicle fuel economy. Higher is better. The EPA sets MPG ratings using a dynamometer test with a fixed driving profile, then adjusts for real-world conditions. The window sticker on every new US car shows three numbers: city MPG, highway MPG, and combined MPG (the weighted average used by the EPA fleet rules).
Real-world MPG — the number a driver gets by dividing trip miles by tank gallons — typically runs 10 to 20 percent below the EPA combined rating. The gap comes from aggressive acceleration, cold starts, headwinds, accessory loads (air conditioning, heated seats), and tire underinflation, none of which the dynamometer test simulates exactly.
The MPG formula
The math is one division: miles divided by gallons. For accuracy, measure from a full tank to a full tank, then average three consecutive tanks to remove single-tank noise from pump shutoff timing and weather.
MPG (US) = miles ÷ gallonsL/100km = 235.215 ÷ MPG (US)km/L = MPG (US) × 0.42514MPG (UK) = MPG (US) × 1.20095Cost/mile = price ÷ MPGAnnual cost = annual miles ÷ MPG × priceA 300-mile trip on 12 gallons works out to 25 MPG. At $3.50 per gallon that trip cost $42, or $0.14 per mile. At 12,000 annual miles, the same MPG produces $1,680 in yearly fuel spending. Bumping the MPG to 35 cuts the annual bill to $1,200 — a $480 saving for a 10-MPG improvement.
MPG to L/100km and km/L
L/100km is the European standard. It measures fuel consumption rather than fuel economy, so lower is better. The conversion to US MPG is 235.215 divided by MPG. The constant combines 3.78541 liters per US gallon with 1.60934 km per mile, scaled by 100 to land on the standard per-100-km figure.
km/L is used in Japan, India, Mexico, and parts of Southeast Asia. The conversion from MPG (US) is multiplication by 0.42514. A 30 MPG sedan does 12.75 km/L. A 50 MPG hybrid does 21.3 km/L.
The CAFE (Corporate Average Fuel Economy) standard, the US federal rule that drove MPG improvements after 1975, uses a fleet-weighted harmonic average rather than a simple arithmetic average. A 50/50 mix of 20 MPG and 40 MPG vehicles averages 26.7 CAFE MPG (not 30), because the harmonic mean weights toward the less-efficient vehicles — the same effect that makes L/100km the more physically meaningful unit.
US MPG vs UK MPG
The same car gets a higher MPG number in the UK because the Imperial gallon is bigger. The US gallon is 3.78541 liters; the Imperial gallon is 4.54609 liters. The Imperial gallon is 20.095% larger, so US MPG times 1.20095 gives UK MPG. A US 30 MPG car is a UK 36 MPG car.
What a good MPG is by vehicle
What counts as a good MPG depends entirely on the class of vehicle. A 28 MPG result is excellent for a midsize pickup and disappointing for a hybrid sedan. The 2024 EPA Automotive Trends Report puts the US new-car fleet average at 28.3 MPG (combined, real-world adjusted), but the spread by class is wide.
- Small sedan 35-40 MPG (Honda Civic, Toyota Corolla)
- Midsize sedan 30-35 MPG (Toyota Camry, Honda Accord)
- Small SUV 27-32 MPG (Honda CR-V, Toyota RAV4)
- Midsize SUV 24-28 MPG (Toyota Highlander, Ford Explorer)
- Full-size pickup 18-22 MPG (Ford F-150, Chevy Silverado)
- Hybrid sedan 50-55 MPG (Toyota Prius, Honda Insight)
- Plug-in hybrid (gas only) 40-55 MPG when battery is depleted
MPG and annual fuel cost
Fuel cost is where MPG translates into real money. The US BLS Consumer Expenditure Survey puts annual household gasoline spending at $2,449 in 2023. At a typical 12,000 annual miles and a $3.50/gal price (AAA early 2026 national average), every 5-MPG improvement saves roughly $200-$400 per year.
A typical hybrid sedan costs $3,000-$5,000 more than the gas equivalent. At a $480/yr fuel saving (10-MPG improvement), the payback is 6-10 years — close to the average length of new-car ownership. Hybrids make financial sense for high-mileage drivers (20,000+ mi/yr) and for drivers who plan to keep the car well past the payback period.
Common MPG measurement mistakes
The most common mistake is dividing odometer miles by tank capacity. The fuel gauge is too imprecise and the manufacturer's tank capacity is the nominal value, not the actual fill. Measure from one click of the pump shutoff to the next, using the trip meter.
The second-most-common mistake is using a single tank. Pump shutoff timing varies by station and nozzle angle, and a 1-gallon overfill or underfill swings a 12-gallon measurement by 8%. Average three tanks for a stable number.
To squeeze the most MPG out of a long highway trip, drop your cruise-control speed by 5-10 mph above 50. The US Department of Energy estimates each 5 mph above 50 mph cuts MPG by about 7%, so dropping from 75 mph to 65 mph improves fuel economy by 12-15% — the largest single behavior change available to most drivers.
One last detail: aggressive acceleration is the biggest single MPG killer for city driving. FuelEconomy.gov cites a 15-30% MPG penalty for aggressive driving on the highway and 10-40% in stop-and-go traffic. Smooth throttle inputs and longer following distances recover most of that.