Article — Fuel Economy Converter (MPG ↔ L/100km)
Fuel economy converter: MPG, L/100km, and the math behind them
To convert miles-per-gallon to liters-per-100-kilometers, divide 235.215 by the MPG value. To go the other way, divide 235.215 by the L/100km value. The same constant works both directions because the two units are reciprocals. So 25 MPG (US) equals 9.41 L/100km, and 8 L/100km equals 29.4 MPG. Higher MPG is better; lower L/100km is better — the scales run opposite directions.
The conversion looks simple but hides a few traps. US and UK MPG use different gallons. The MPG scale is nonlinear, so a 5-MPG bump means very different things at the low and high ends. Electric vehicles use an entirely different unit (MPGe). Each of these gets a section below.
What is fuel economy?
Fuel economy measures how far a vehicle travels per unit of fuel consumed. In the US, the standard unit is miles per US gallon (MPG). In most of Europe and Australia, it is liters per 100 kilometers (L/100km). Japan and South Korea use kilometers per liter (km/L). All three describe the same physical quantity, just inverted or scaled differently.
The US has measured cars in MPG since the Energy Policy and Conservation Act of 1975, which set the first Corporate Average Fuel Economy (CAFE) standards in response to the 1973 oil shock. The new-car fleet was averaging 13.5 MPG; the law set a target of 27.5 MPG by 1985. The current 2026 model-year standard is around 45 MPG combined, with a 55-MPG target for 2032.
The first US fuel-economy labels in 1975 listed only city and highway MPG. The EPA added a combined figure in 1985, a gallons-per-100-miles figure in 2013 (to fight the MPG illusion), and an MPGe figure in 2011 for electric vehicles. Today's label, redesigned in 2013, shows annual fuel cost, MPG by mode, and a fuel-savings projection over five years against the average car.
The MPG to L/100km formula
The conversion uses one constant: 235.215. Divide that by MPG to get L/100km, or divide it by L/100km to get MPG. The constant comes from straight unit arithmetic — 1 US gallon equals 3.78541 liters, 1 mile equals 1.60934 kilometers. Working through: (100 km / 1.60934 km/mi) × (1 / MPG) × 3.78541 L/gal = 235.215 / MPG.
L/100km = 235.215 / MPG MPG = 235.215 / L/100km1 US gal = 3.785 L 1 UK gal = 4.546 LRounding the constant to 235 keeps the math doable in your head. A 30-MPG car is 235 / 30 ≈ 7.83 L/100km (true: 7.84). A 50-MPG hybrid is 235 / 50 = 4.7 L/100km. A 15-MPG pickup is 235 / 15 ≈ 15.7 L/100km. The error from rounding 235.215 to 235 is under 0.1% — invisible for any practical purpose.
The MPG illusion (why higher is not linearly better)
Comparing cars by MPG misleads in a specific way. Replacing a 15-MPG truck with a 20-MPG truck saves more fuel than replacing a 30-MPG sedan with a 50-MPG hybrid — yet the second swap looks bigger on paper. The reason is that MPG is distance-per-volume, but fuel cost and emissions scale with volume-per-distance.
Run the numbers. Driving 10,000 miles at 15 MPG uses 667 gallons. At 20 MPG, the same 10,000 miles uses 500 gallons — a saving of 167 gallons. At 30 MPG, 10,000 miles uses 333 gallons. At 50 MPG, only 200 gallons — saving 133 gallons. The first swap (the smaller MPG bump) saves more fuel because it kills off the worst end of the curve.
Replacing a 15-MPG SUV with a 25-MPG sedan saves about 267 gallons per 10,000 miles. Replacing a 35-MPG car with a 50-MPG hybrid saves only 86 gallons. The first swap is three times more effective at cutting fuel use, even though both are "10–15 MPG improvements". Always think in gallons per 100 miles, not MPG, when sizing the impact of a vehicle change.
The EPA added a "gallons per 100 miles" figure to fuel-economy labels in 2013 precisely to surface this. The metric scales linearly with consumption: a car using 3 gal/100 mi consumes exactly half the fuel of one using 6 gal/100 mi. Europe and Australia have used L/100km since the 1970s for the same reason.
US MPG versus UK (imperial) MPG
The UK and Canada historically used the imperial gallon (4.546 liters), which is about 20% larger than the US gallon (3.785 liters). A car that gets 25 US-MPG is rated 30 UK-MPG without any change in actual fuel use. The math: UK MPG = US MPG × 1.20095.
- 1 US gallon = 3.78541 liters = 128 US fluid ounces
- 1 imperial gallon = 4.54609 liters = 160 imperial fluid ounces
- UK MPG / US MPG ratio = 1.20095 (the gallon ratio)
- 25 US-MPG = 30.0 UK-MPG = 9.41 L/100km
- 40 US-MPG = 48.0 UK-MPG = 5.88 L/100km
- Canada uses US-style L/100km officially, but older publications quote imperial MPG
- UK switch — official UK car listings now show L/100km alongside imperial MPG (since 2001)
The confusion costs money. A traveler comparing a 40-MPG UK car ad to a 40-MPG US car ad sees the same number, but the UK car uses 20% more fuel per mile. Always confirm which gallon is meant. Modern automaker websites usually clarify with an asterisk; classic car listings rarely do.
Fuel economy by vehicle class
The EPA's Automotive Trends Report tracks fleet-wide fuel economy by class. Model-year 2023 averages (combined city and highway, real-world adjusted):
Sedans lead non-hybrid gasoline cars at 30.7 MPG (7.66 L/100km). Car-style SUVs (smaller, unibody crossovers) come in at 29.9 MPG. Truck-style SUVs (larger, body-on-frame like Tahoe and Suburban) drop to 24.3 MPG. Pickups are the worst class at 20.5 MPG, dragged down by full-size half-tons and heavy-duty work trucks.
Hybrids change the picture. The 2024 Toyota Prius is rated 57 MPG combined (4.13 L/100km). The Hyundai Elantra Hybrid hits 54 MPG. Plug-in hybrids in pure-gas mode usually do 40–50 MPG; in blended electric-plus-gas operation, they post 70–100 MPGe. Pure EVs typically run 100–140 MPGe equivalent, with the Lucid Air at the top of the range at 130+ MPGe.
MPGe: how the EPA rates electric vehicles
Electric vehicles do not burn gasoline, so MPG does not apply directly. The EPA solved this in 2011 by defining MPGe — miles per gallon equivalent. One gallon of gasoline contains 33.7 kWh of chemical energy (the EPA's "energy equivalent of one gallon of gasoline"). An EV that travels 100 miles on 28.1 kWh is rated 100 × (33.7 / 28.1) = 120 MPGe.
The conversion lets buyers compare EVs and gasoline cars on the same scale. A Tesla Model 3 Long Range is rated 132 MPGe — equivalent to a gasoline car getting 132 MPG. A Toyota RAV4 hybrid is rated 40 MPG. In raw energy terms, the EV uses about a third of the fuel-equivalent energy for the same distance.
MPGe does not account for electricity costs at the wall, which vary by region. At 12¢/kWh, a 120-MPGe EV costs about $3.40 per 100 miles; at $3.50/gal gasoline, a 30-MPG car costs $11.67 per 100 miles. The EV is roughly 3–4× cheaper to run, even though MPGe only shows a 4× energy advantage on the sticker.
Improving real-world fuel economy
EPA window-sticker numbers describe a controlled laboratory test. Real-world fuel economy is typically 10–20% lower because real driving includes aggressive accelerations, cold starts, air conditioning, highway speeds above the test average of 48–60 mph, and elevation changes. The good news: most of that gap is recoverable through driving habits.
The biggest lever is speed. Each 5 mph above 60 mph costs roughly 7% in fuel economy, mostly to wind drag (which scales with the square of speed). A car rated 30 MPG at 60 mph might drop to 25 MPG at 75 mph. The second-biggest lever is acceleration — hard launches cut MPG by 15–30% in the short term and never fully recover.
Tire pressure matters more than people think. Under-inflated tires (5 PSI below spec) cost 1–2 MPG by adding rolling resistance. Modern cars with tire-pressure monitoring will warn you, but the warning kicks in at 25% low, which is well past the efficiency loss. Check pressures monthly with a gauge, not the dashboard light.