Fuel efficiency measures how effectively a vehicle converts fuel into distance traveled.
Units
- MPG (US/UK): Miles per gallon — higher is better
- L/100km (metric): Liters per 100 kilometers — lower is better
- km/L (Japan, India): Kilometers per liter — higher is better
Conversion
MPG to L/100km: Divide 235.215 by MPG. Example: 30 MPG = 7.84 L/100km.
Factors Affecting Efficiency
Speed (optimal 45–65 mph), tire pressure, weight, aerodynamics, driving habits, maintenance, weather.
Why MPG Is a Misleading Scale
Improving from 10 to 20 MPG saves far more fuel over 10,000 miles than improving from 40 to 50 MPG, even though both represent the same "10 MPG" gain — the relationship between MPG and actual gallons used is not linear. This is a key reason many countries outside the U.S. use gallons/liters per 100 miles/km instead, which scales linearly with actual fuel consumption.
Real-World vs. EPA-Rated MPG
EPA fuel economy ratings are measured under controlled laboratory conditions and tend to overstate real-world city driving efficiency, where stop-and-go traffic, cold starts, and accessory use (AC, heat) all reduce actual MPG below the sticker rating — drivers commonly see 10–20% lower real-world mileage than the EPA city estimate.