Article — Fuel Mileage Calculator
Fuel mileage: turning distance and fuel into a number
Fuel mileage is the distance a vehicle travels per unit of fuel. In the US it is expressed as miles per gallon (MPG); in most of the world, as litres per 100 kilometres (L/100km). A typical 2024 compact sedan averages 32 MPG (US), which equals 7.4 L/100km.
The calculator takes the two raw numbers you can measure at the pump — distance since last fill-up, fuel needed to fill the tank — and converts them into every common fuel-economy figure. That lets you compare your real-world results to the EPA window sticker, to the European NEDC or WLTP number, or to a friend's car in a different country.
What fuel mileage means
It is just a ratio. Pick any unit of distance and any unit of fuel, and the ratio tells you how efficiently the car turns fuel into motion. MPG and L/100km are inverse ratios — one is distance over fuel, the other is fuel over distance — so they move in opposite directions when economy improves.
Real-world fuel mileage averages 8-15 percent below the EPA window sticker for most modern cars, according to EPA's 2024 Fuel Economy Trends report. The gap widens in cold weather, short trips, and aggressive driving. To get a meaningful number, average several fill-ups over at least 500 km / 300 miles.
The first US fuel-economy standards were set by the Energy Policy and Conservation Act of 1975, after the 1973 oil crisis. New-car average fuel economy was 13 MPG in 1973; by 2024 it had reached about 27 MPG combined, despite vehicles getting heavier and faster.
How to calculate fuel mileage
The reset-trip method gives a clean reading. Fill the tank until the pump clicks off the first time. Reset the trip odometer. Drive normally for at least 200 miles or 300 km. Refill until the pump clicks off the first time again, and write down both the trip distance and the litres or gallons it took. Divide.
MPG = miles / gallons L/100km = L × 100 / kmkm/L = km / L L/100km = 235.215 / MPG (US)MPG (UK) = MPG (US) × 1.20095Use the same procedure for several tanks and average the results. A single tank can vary by 10-15 percent from one fill-up to the next because of how full the pump leaves the tank.
MPG vs L/100km
MPG goes up when fuel economy improves; L/100km goes down. That single difference produces a well-documented cognitive trap called the MPG illusion. Going from 15 to 18 MPG saves more fuel over a year than going from 30 to 40 MPG, because the change in actual fuel consumption is bigger at low MPG values. In L/100km the same trade-offs look more intuitive.
Good fuel mileage by vehicle class
For a 2024 gasoline car, EPA combined ratings cluster around:
- Subcompact car — 30-36 MPG (6.5-7.8 L/100km)
- Compact sedan — 28-34 MPG (6.9-8.4 L/100km)
- Midsize sedan — 26-32 MPG (7.4-9.0 L/100km)
- Compact SUV — 25-30 MPG (7.8-9.4 L/100km)
- Midsize SUV — 21-26 MPG (9.0-11.2 L/100km)
- Full-size pickup — 17-22 MPG (10.7-13.8 L/100km)
- Compact hybrid — 48-58 MPG (4.1-4.9 L/100km)
- Plug-in hybrid — 40-90 MPGe combined
What drops fuel mileage
Cold weather cuts fuel economy 10-20 percent. The engine warms up slowly, oil is thicker, tires lose pressure (about 1 psi per 10 deg F drop), and cabin heat draws extra load. Stop-and-go traffic costs another 15-25 percent versus steady highway driving.
Speed matters too. At 50 mph most cars hit peak fuel efficiency. Above that, aerodynamic drag rises with the square of speed. EPA estimates fuel economy falls 7-14 percent for every 10 mph above 50. Roof racks add 5-25 percent fuel cost on the highway.
Modern fuel-injected engines use almost no extra fuel to restart, but burn 0.2-0.7 gallons per hour idling. If you will be stopped more than 10 seconds (and traffic is not the reason), turn off the engine. Idling for 5 minutes a day for a year burns roughly 30 gallons.
Improving fuel mileage
The biggest wins come from steady speed and tire pressure. Cruise control on highway sections, tires inflated to the door-sticker spec (not the maximum on the sidewall), regular air-filter changes, and reducing dead weight in the trunk all add up to 10-15 percent better fuel mileage with no other changes. Combine them and a 30-MPG car can sustain 33-35 MPG without trying.
Use the cheapest grade of gasoline your owner manual permits. Higher-octane fuel only improves mileage if the engine is tuned for it. A premium-required car run on regular can knock and lose power; a regular-only car run on premium gains nothing measurable.
MPGe for EVs and hybrids
EPA created MPGe (miles per gallon equivalent) to compare electric and gasoline vehicles. The conversion is 33.7 kWh = 1 gallon of gasoline equivalent, based on chemical energy content. So a Tesla Model 3 rated 132 MPGe uses 33.7/132 = 0.255 kWh per mile, or about 25.5 kWh per 100 miles.
Plug-in hybrids show two ratings — one for electric-only driving (typically 80-110 MPGe) and one for gasoline-only mode (35-50 MPG). The combined sticker number is a weighted average based on assumed daily mileage and charging behaviour.
Common mistakes
Mixing US and UK gallons is the most common error. UK gallons are 20 percent bigger, so a British 36 MPG car only delivers 30 MPG when measured by US gallons. Using the EPA highway number alone is another trap — most people drive in mixed conditions and the combined rating is more honest. Finally, comparing your real-world result to the window sticker without averaging multiple tanks will give noisy answers; a single tank can be off by 10-15 percent from your true mileage.
A subtler error is testing fuel mileage on a single highway run and assuming that figure represents your everyday driving. Best-case highway numbers can be 30-50 percent higher than your typical commute. The EPA combined rating already weights city and highway 55/45, which lines up with the average driver's mix. To get a number you can trust, average five or six fill-ups across your normal routes.
The last trap is ignoring fuel mileage entirely because the numbers feel small. A 5-MPG improvement on a 12000-mile-per-year commute saves 60-90 gallons annually. At $3.50 a gallon that is $200-300 a year, and over a decade of car ownership it is the difference between paying for a couple of tanks of fuel and paying for a small vacation.