The Physics and Economics of Fuel Consumption: MPG vs. L/100km
In automotive engineering and international car buying, comparing fuel efficiency standards across borders requires navigating two fundamentally opposed mathematical paradigms. The United States and United Kingdom measure distance per volume (Miles per Gallon), whereas continental Europe, Canada, Australia, and New Zealand measure volume consumed per distance (Liters per 100 Kilometers).
1. The Mathematical Conversion Proofs
Converting between Miles per Gallon and Liters per 100 km requires reconciling both the imperial-to-metric length conversion (1 mile = 1.609344 km) and the liquid volume conversion (1 US gallon = 3.785411784 liters):
Fundamental Fuel Efficiency Equations:
1. L/100km = (100 × 3.785411784) ÷ (1.609344 × US MPG) = 235.214583 ÷ US MPG
2. US MPG = 235.214583 ÷ L/100km
3. UK Imperial MPG = US MPG × 1.20095 = 282.481 ÷ L/100km
4. km per Liter (km/L) = 100 ÷ L/100km = US MPG × 0.425144
2. The "MPG Illusion": Why Higher MPG Can Deceive Drivers
In a landmark behavioral economics paper published in Science, Dr. Richard Larrick and Dr. Jack Soll of Duke University identified the cognitive bias known as the "MPG Illusion". Because MPG expresses efficiency as an inverse curve ($y = 1/x$), equal increments in MPG do not translate to equal reductions in fuel burned.
Consider two drivers replacing their vehicles for a 10,000-mile annual commute:
- Driver A: Upgrades a full-size pickup from 14 MPG to 20 MPG (+6 MPG gain). Gas consumed drops from 714 gallons to 500 gallons—a net savings of 214 gallons ($749/year at $3.50/gal).
- Driver B: Upgrades a compact car from 33 MPG to 50 MPG (+17 MPG gain). Gas consumed drops from 303 gallons to 200 gallons—a net savings of only 103 gallons ($360/year at $3.50/gal).
Even though Driver B gained nearly three times as many MPG points (+17 vs. +6), Driver A saved more than double the actual gasoline and cash. The metric system's L/100km (or Gallons per 100 Miles) avoids this trap because it scales linearly with true consumption.
3. Real-World Road Trip Fuel Budgeting
When planning highway travel across interstate corridors, drivers should account for aerodynamic drag at high speeds. EPA highway fuel ratings are tested at an average speed of 48.3 mph with a peak of 60 mph. Traveling at typical Western interstate speeds (75 mph to 80 mph) increases aerodynamic drag exponentially, degrading highway MPG by 15% to 25%.