Fuel Consumption Calculator

Fuel Economy: MPG, L/100km, and km/L

Three units dominate global fuel-economy reporting, and converting between them trips up drivers, fleet managers, and rental-car shoppers every day. Miles per gallon (MPG) is used almost exclusively in the United States and the United Kingdom, where higher numbers are better. Litres per 100 km (L/100km) is the continental European and Canadian standard, where lower numbers are better. Kilometres per litre (km/L) is common in Japan, India, and much of Latin America, where higher is better. The US gallon and the Imperial (UK) gallon are different sizes — 3.785 L vs 4.546 L — so a car rated 40 mpg in a US brochure is really 48 mpg UK. This calculator uses the US gallon by default.

The mathematics matter: MPG is a reciprocal relationship with L/100km, not a linear one. Improving a car from 15 mpg to 20 mpg saves more fuel over 10 000 miles than improving another from 30 mpg to 40 mpg, even though the second jump looks larger. This is called the "MPG illusion" and is why European governments report L/100km — it avoids the illusion by putting fuel on top of the fraction.

Why "Mixed Driving" Ratings Exist

EPA, WLTP, and NEDC test cycles all report separate city, highway, and combined figures because real-world fuel economy depends heavily on driving pattern. At highway cruise (55–65 mph), aerodynamic drag dominates and modern cars achieve their best numbers. In city driving, stop-and-go idling and repeated acceleration from a standstill erase efficiency — hybrids shine here because regenerative braking recovers energy that conventional cars dump as heat. Cold weather can drop winter fuel economy 10–20% because engines run rich until warm and cabin heating costs energy. Your real-world MPG will almost always be 5–15% below the sticker figure.

Converting Between Units

Worked Examples

Example 1 — A US road trip. You drive 420 miles and fill up with 14.5 US gallons. MPG = 420 / 14.5 = 29.0 mpg. L/100km = 235.215 / 29 = 8.1 L/100km. At $3.50/gal, the trip cost 14.5 × $3.50 = $50.75, or about $0.121 per mile.

Example 2 — European commute. A diesel hatchback uses 5.2 L/100km over 900 km. Total fuel = 5.2 × 9 = 46.8 L. At €1.75/L, the trip costs €81.90. In US units that is 235.215 / 5.2 = 45.2 mpg — excellent by US standards.

Example 3 — The MPG illusion, quantified. Two households each drive 12 000 miles per year. Household A upgrades from a 15 mpg SUV to a 20 mpg SUV: fuel use drops from 800 to 600 gal — saves 200 gal. Household B upgrades from a 30 mpg sedan to a 40 mpg sedan: fuel use drops from 400 to 300 gal — saves only 100 gal. The smaller MPG jump saved twice the fuel.

Example 4 — Cost per mile for budgeting. A pickup averages 18 mpg at $4.00/gal. Cost per mile = $4.00 / 18 = $0.222. Over 15 000 miles/year that is $3 333 in fuel. A hybrid at 50 mpg at the same price is $0.080/mile = $1 200/year — a $2 133 annual difference, which justifies a significant price premium on the hybrid over a few years.

Common Pitfalls

• Trip computer optimism. The dashboard MPG display is typically 2–5% more optimistic than the measured-at-the-pump value. Use the pump method (miles driven divided by gallons to refill) for accurate tracking.
• Mixing US and UK gallons. A UK brochure claiming 50 mpg is only 41.6 mpg US. Always check which gallon the quote refers to.
• Averaging MPG across trips. You cannot average MPG figures directly. To combine, convert each to total fuel used, add, then divide by total distance.
• Ignoring fuel grade differences. Premium gas has similar energy content to regular; in a car that does not require it, you get no measurable economy improvement — just a higher fuel bill.
• Not warming up correctly. Modern engines warm up faster while driving gently than idling. Long idle warm-ups waste fuel and distort short-trip MPG readings.

Frequently Asked Questions

Why is my real-world MPG lower than the sticker? EPA and WLTP cycles are run in controlled conditions (mild temperature, steady driver, flat track). Real-world driving involves cold starts, headwinds, traffic, cargo, and individual driving style. A 10% gap is normal; 20%+ suggests a mechanical issue or very aggressive driving.

Do tire pressure and oil really matter? Yes. Underinflated tires by 5 psi can cost 2–3% in fuel economy; the wrong oil viscosity can cost another 1–2%. These are among the cheapest MPG improvements available.

Does air conditioning really hurt MPG? A/C can draw 1–4 horsepower, worth 5–10% of fuel economy at low speeds. At highway speeds the penalty is smaller than opening windows, which adds drag. Above about 45 mph, A/C beats windows down.

Is hypermiling worth it? Techniques like smooth acceleration, coasting to lights, and steady highway speed can improve economy 15–30% with no equipment changes. The biggest gains come from reducing cruising speed from 75 to 65 mph — often a 10–15% saving.

How do EVs compare? EVs use MPGe (miles per gallon equivalent) based on energy content: 33.7 kWh = 1 US gallon of gasoline. A Tesla Model 3 rated 132 MPGe uses 33.7 / 132 × 100 = 25.5 kWh/100 mi. At $0.15/kWh that is $3.83 per 100 miles — typically a third to half the fuel cost of a comparable gasoline car.

Related Calculators

Planning a road trip? Use the Speed / Distance / Time Calculator to estimate drive time and the Unit Price Comparer to spot the cheapest fuel. Comparing vehicle loans? The Loan Calculator pairs with monthly fuel cost to get real total-cost-of-ownership. Running percentage comparisons on MPG improvements? The Percentage Calculator handles it. Browse the Mathematics category for more.

Disclaimer

This calculator is provided for educational and informational purposes only. While we strive for accuracy, users should verify all calculations independently. We are not responsible for any errors, omissions, or damages arising from the use of this calculator.