Fuel Economy: MPG, L/100km and Why They Tell Different Stories

The Upgrade That Saves Less Fuel Than You Think

Upgrading a car from 10 MPG to 20 MPG saves 50 gallons over every 1,000 miles. Upgrading a different car from 30 MPG to 40 MPG saves only 8.3 gallons over the same 1,000 miles. The second upgrade sounds more impressive as a headline number, but it delivers six times less real-world fuel savings. This is the MPG illusion, and it catches out car buyers, fleet managers, and policy makers every year.

The illusion exists because MPG is the inverse of fuel consumption. The relationship is not linear. Moving from bad fuel economy to mediocre fuel economy cuts far more actual fuel use than moving from good to excellent. L/100km, the unit used in Europe, Canada, and Australia, avoids the illusion entirely because it measures consumption directly: liters burned per 100 kilometers traveled. Lower is always better, and the improvements are proportional.

This guide covers the conversion between MPG and L/100km, how the EPA generates its city/highway/combined ratings, the actual relationship between speed and fuel economy (the highway is not always optimal), electric vehicle efficiency expressed in MPGe, and the full cost-per-mile calculation that tells you what you spend at the pump.

MPG, L/100km, and the Inverse Relationship

Miles per gallon (MPG) measures how far a vehicle travels on one gallon of fuel. The higher the number, the more distance per unit of fuel. A car rated 30 MPG travels 30 miles on one gallon. A car rated 15 MPG travels only 15 miles on the same gallon.

Liters per 100 kilometers (L/100km) measures how many liters of fuel a vehicle burns to travel 100 kilometers. Lower is better: 5 L/100km burns less fuel than 8 L/100km over the same distance.

The two units are mathematical inverses of each other, using a conversion constant of 235.21 (the number of kilometers in a gallon of fuel at 1 L/100km). The formula:

L/100km = 235.21 / MPG

Examples: 30 MPG = 235.21 / 30 = 7.84 L/100km. 40 MPG = 235.21 / 40 = 5.88 L/100km. 20 MPG = 235.21 / 20 = 11.76 L/100km. 50 MPG = 235.21 / 50 = 4.70 L/100km.

UK MPG uses the Imperial gallon (4.546 liters), not the US gallon (3.785 liters). A car rated 40 UK MPG equals about 33 US MPG. Never compare UK and US MPG figures directly.

The Math Behind Fuel Cost Per Mile

Fuel cost per mile gives you the real-world number for budgeting. Three values go in:

1. Fuel price (dollars per gallon, or local currency per liter)
2. Fuel economy (MPG or L/100km)
3. Distance driven

Cost per mile formula: Cost per mile = Fuel price / MPG

Example: fuel costs $3.50 per gallon, car gets 28 MPG. Cost per mile = $3.50 / 28 = $0.125 per mile. Over 15,000 miles per year: $0.125 × 15,000 = $1,875 per year in fuel.

Same driver upgrades to a 40 MPG hybrid: $3.50 / 40 = $0.0875 per mile. Annual cost: $0.0875 × 15,000 = $1,313 per year. Savings: $562 per year.

In L/100km terms: cost per 100 km = L/100km × price per liter. At 8 L/100km and $1.60/liter: 8 × 1.60 = $12.80 per 100 km. Over 20,000 km/year: $12.80 × 200 = $2,560 per year.

Common Misconceptions

• Myth: Highway driving always gives the best fuel economy. EPA highway ratings assume speeds of 48 to 80 mph. At 75 mph, aerodynamic drag increases significantly compared to 55 mph. Most gasoline vehicles hit peak fuel economy between 45 and 55 mph. Driving at 75 mph instead of 55 mph reduces fuel economy by roughly 15 to 20 percent.
• Myth: EPA ratings are what you'll get in real life. The EPA combined rating comes from lab testing on a dynamometer. Real-world results typically run 10 to 20 percent below the combined label. The EPA's own website now posts real-world adjustment factors and shows the range drivers report.
• Myth: Air conditioning has a tiny effect on MPG. Air conditioning can reduce fuel economy by 5 to 25 percent, depending on ambient temperature, humidity, and whether the vehicle has an efficient compressor. At city speeds in summer heat, the impact sits at the high end. At highway speeds, it is proportionally smaller.
• Myth: Electric vehicles don't have a fuel economy metric. The EPA rates EVs in MPGe (miles per gallon equivalent), where 33.7 kWh of electricity equals one gallon of gasoline in energy content. A car rated 120 MPGe travels the equivalent distance on the energy in one gasoline gallon. Typical EVs score between 100 and 140 MPGe.