Fuel Efficiency Explained: MPG, L/100km, and km/L Conversions Introduction Fuel efficiency is a core concept in transportation, engineering, and everyday life. It describes how far a vehicle will travel for a given amount of fuel, or conversely, how much fuel is required to travel a certain distance. Different parts of the world use different units to express fuel efficiency—most commonly miles per gallon (MPG) in the United States and the United Kingdom, liters per 100 kilometers (L/100km) in much of the world, and kilometers per liter (km/L) in some countries. Understanding how to convert between these units is essential for comparing vehicles, estimating fuel costs, designing fuel systems, or calculating greenhouse gas emissions. This article explains the key concepts, provides exact conversion formulas (including the distinction between US and Imperial gallons), gives worked numerical examples, and highlights practical engineering, scientific, and everyday use cases. It also offers tips and best practices to avoid common mistakes. Key Concepts/Definitions Fuel efficiency: A general term indicating how much distance a vehicle travels per quantity of fuel (efficiency) or how much fuel is consumed per unit distance (consumption). Note that terminology can be confusing because "higher is better" for MPG and km/L, whereas "lower is better" for L/100km. MPG (miles per gallon): Distance traveled in miles per gallon of fuel. There are two gallon definitions commonly used: US gallon = 3.785411784 liters Imperial (UK) gallon = 4.54609 liters Make sure you know which gallon is referenced when comparing MPG values. L/100km (liters per 100 kilometers): Fuel consumed in liters for every 100 kilometers traveled. This is a direct consumption measure: lower values indicate better fuel economy. km/L (kilometers per liter): Distance in kilometers traveled per liter of fuel. This is similar to MPG conceptually but uses metric distance and volume. higher values indicate better fuel economy. Inversion relationship: km/L and L/100km are reciprocals scaled by 100: L/100km = 100 / (km/L) km/L = 100 / (L/100km) Understanding these definitions ensures conversions are interpreted correctly and comparisons are valid. Conversion Formulas Start with the exact base conversions: 1 mile = 1.609344 kilometers 1 US gallon = 3.785411784 liters 1 Imperial gallon = 4.54609 liters From those, derive conversion formulas. 1) Convert MPG (US) to km/L: km/L = mpg_US × (1.609344 km/mile) / (3.785411784 L/gallon) Numerical constant: 1.609344 / 3.785411784 = 0.425143707 So: km/L = mpg_US × 0.425143707 2) Convert MPG (US) to L/100km: Use the reciprocal of km/L and scale by 100: L/100km = 100 / (km/L) = 100 / (mpg_US × 0.425143707) Simplified constant: L/100km = 235.214583 / mpg_US Common rounded constant you may see: 235.215 / mpg (US) 3) Convert km/L to MPG (US): Invert the constant above: mpg_US = km/L × (3.785411784 / 1.609344) Numerical constant: 3.785411784 / 1.609344 = 2.352145833 So: mpg_US = km/L × 2.352145833 4) Convert L/100km to MPG (US): mpg_US = 235.214583 / (L/100km) 5) For Imperial (UK) MPG, use the Imperial gallon: km/L = mpg_Imp × (1.609344 / 4.54609) = mpg_Imp × 0.35400619 L/100km = 100 / (mpg_Imp × 0.35400619) = 282.480936 / mpg_Imp mpg_Imp = km/L × (4.54609 / 1.609344) = km/L × 2.824809364 mpg_Imp = 282.480936 / (L/100km) Summary of key formulas (bolded for emphasis): km/L = mpg_US × 0.425143707 L/100km = 235.214583 / mpg_US mpg_US = km/L × 2.352145833 mpg_US = 235.214583 / (L/100km) And for Imperial: km/L = mpg_Imp × 0.35400619 L/100km = 282.480936 / mpg_Imp mpg_Imp = km/L × 2.824809364 mpg_Imp = 282.480936 / (L/100km) Conversion Reference Table Below is a quick reference table converting common US mpg values into km/L and L/100km. (For Imperial MPG, use the formulas above or see the notes.) | mpg (US) | km/L (≈) | L/100km (≈) | |----------:|---------:|------------:| | 10 | 4.251 | 23.52 | | 15 | 6.377 | 15.68 | | 20 | 8.503 | 11.76 | | 25 | 10.629 | 9.41 | | 30 | 12.754 | 7.84 | | 35 | 14.880 | 6.72 | | 40 | 17.006 | 5.88 | | 45 | 19.131 | 5.23 | | 50 | 21.257 | 4.70 | Notes: Values are rounded to three decimal places for clarity. To convert a value shown in Imperial MPG, use L/100km = 282.480936 / mpg_Imp or recalculate km/L using the Imperial constant. Practical Examples (with worked calculations) Below are step-by-step examples converting between common units. These show how to apply the formulas precisely. Example 1 — Convert 30 mpg (US) to km/L and L/100km: 1. Use km/L = mpg_US × 0.425143707 km/L = 30 × 0.425143707 = 12.75431121 km/L Round to sensible precision: 12.754 km/L 2. Convert km/L to L/100km: L/100km = 100 / (km/L) L/100km = 100 / 12.75431121 = 7.843137255 L/100km Rounded: 7.84 L/100km So 30 mpg (US) ≈ 12.75 km/L ≈ 7.84 L/100km. Example 2 — Convert 8.0 L/100km to mpg (US) and km/L: 1. Convert to km/L: km/L = 100 / (L/100km) km/L = 100 / 8.0 = 12.5 km/L 2. Convert km/L to mpg (US): mpg_US = km/L × 2.352145833 mpg_