mpg, L/100 km and kWh/100 km explained

Evaluating automotive efficiency across international markets requires navigating fundamentally different measurement philosophies. In the United States and the United Kingdom, drivers assess vehicle efficiency in distance traveled per unit of fuel consumed. In continental Europe and many other metric regions, drivers evaluate efficiency as the volume of fuel or energy required to travel a standardized fixed distance.

Comparing vehicles accurately demands understanding how these metrics function, how regional definitions diverge, and why direct conversions require exact arithmetic formulas. Standardized calculation rules, such as those detailed in the site's measurement methodology, ensure that energy data remains consistent across borders.

Distance-based economy versus consumption-based efficiency

Vehicle efficiency divides into two mathematical paradigms: distance-per-volume economy and volume-per-distance consumption.

Distance per unit of fuel (Economy)

Metrics such as miles per gallon express distance achieved per unit of fuel. Higher numerical values indicate superior efficiency. According to the U.S. Environmental Protection Agency and Department of Energy on FuelEconomy.gov, some form of the miles per gallon metric has been on vehicle labels since 1977.

When evaluating combustion vehicles in the United States, official window stickers display City, Highway, and Combined MPG values. FuelEconomy.gov explains that combined fuel economy is a weighted average of City and Highway MPG values that is calculated by weighting the City value by 55% and the Highway value by 45%. The label also assigns each vehicle a rating from 1 (worst) to 10 (best) for fuel economy and greenhouse gas emissions, assuming an annual mileage of 15,000 miles.

Volume or energy per fixed distance (Consumption)

Metric jurisdictions evaluate vehicles through consumption metrics: liters per 100 kilometers for liquid fuels, or kilowatt-hours per 100 kilometers for electric drivetrains. Lower numerical values indicate superior efficiency because less fuel or energy is consumed to cover the benchmark distance.

The MPG illusion: why linear thinking misleads drivers

Distance-per-volume metrics introduce a cognitive trap known as the MPG illusion. Because miles per gallon places fuel volume in the denominator, the relationship between MPG improvements and actual fuel saved is non-linear.

Technical documentation from FuelEconomy.gov explains this phenomenon directly:

While a miles per gallon (MPG) estimate is a required feature that has appeared on the fuel economy label for several decades, this metric can be potentially misleading when consumers compare fuel economy improvements, particularly when they use it in place of fuel costs. The fuel savings, in gallons, for a vehicle that gets 10 MPG versus a vehicle that gets 15 MPG is about 33 gallons (assuming 1,000 miles). On the other hand, the fuel savings in gallons, for the same 5 MPG fuel economy jump, for a 30 MPG versus a 35 MPG vehicle is only about 5 gallons.

FuelEconomy.gov notes that this MPG illusion demonstrates why it may be more meaningful to express fuel efficiency in terms of consumption (such as gallons per 100 miles or liters per 100 kilometers) rather than in terms of economy. In a consumption-based metric, every unit reduction translates to an identical reduction in fuel consumed and money spent, regardless of the vehicle baseline.

The two gallons: US liquid gallon versus Imperial gallon

A frequent source of cross-border confusion involves the word gallon itself. The United States and the United Kingdom use distinct legal definitions of the gallon:

Under standard physical definitions published by the National Institute of Standards and Technology (NIST Special Publication 811):

• 1 US gallon = 3.785411784 L
• 1 imperial gallon = 4.54609 L
• 1 mile = 1.609344 km

Because an imperial gallon holds approximately 1.20095 times the volume of a US liquid gallon, a vehicle tested over identical driving conditions will display a higher numerical rating in British publications than in American publications. A rating in mpg (UK) is not directly comparable to mpg (US) without arithmetic conversion.

Conversion formulas for liquid fuel metrics

Converting between distance-per-volume and volume-per-distance requires exact arithmetic formulas derived from international standards.

1. Converting between mpg (US) and L/100 km

To derive the relationship between US miles per gallon and L/100 km, convert distance and volume using NIST conversion constants:

1 US gallon contains 3.785411784 liters, and 1 mile equals 1.609344 kilometers.

The formula to convert from mpg (US) to L/100 km is:

$$\text{L/100 km} = \frac{100 \times 3.785411784}{1.609344 \times \text{mpg (US)}} \approx \frac{235.214583}{\text{mpg (US)}}$$

Conversely, to convert from L/100 km to mpg (US):

$$\text{mpg (US)} = \frac{235.214583}{\text{L/100 km}}$$

2. Converting between mpg (UK) and L/100 km

Using the imperial gallon definition of 4.54609 liters and 1.609344 kilometers per mile, the formula to convert from mpg (UK) to L/100 km is:

$$\text{L/100 km} = \frac{100 \times 4.54609}{1.609344 \times \text{mpg (UK)}} \approx \frac{282.480936}{\text{mpg (UK)}}$$

Conversely, to convert from L/100 km to mpg (UK):

$$\text{mpg (UK)} = \frac{282.480936}{\text{L/100 km}}$$

3. Converting between mpg (US) and mpg (UK)

To convert between American and British fuel economy figures directly:

$$\text{mpg (UK)} = \text{mpg (US)} \times \frac{4.54609}{3.785411784} \approx \text{mpg (US)} \times 1.20095$$

$$\text{mpg (US)} = \text{mpg (UK)} \times \frac{3.785411784}{4.54609} \approx \text{mpg (UK)} \times 0.832674$$

Electric vehicle consumption metrics

Electric vehicle efficiency introduces electrical energy units to quantify energy drawn from the grid or battery.

1. Kilowatt-hours per 100 kilometers (kWh/100 km)

In metric jurisdictions, electric vehicle efficiency is stated in kWh/100 km. Like L/100 km for gasoline, kWh/100 km represents a consumption metric where lower numerical values signify higher electrical efficiency.

2. Kilowatt-hours per 100 miles (kWh/100 mi)

The United States EPA utilizes kWh/100 mi on official electric vehicle window labels. Because 1 mile equals 1.609344 kilometers:

$$\text{kWh/100 mi} = \text{kWh/100 km} \times 1.609344$$

$$\text{kWh/100 km} = \frac{\text{kWh/100 mi}}{1.609344} \approx \text{kWh/100 mi} \times 0.621371$$

Drivers can calculate charging expenses using the charging cost tool to observe how regional electricity rates translate these consumption ratings into monetary expenses.

3. Miles per kilowatt-hour (mi/kWh)

In the United Kingdom, electric vehicle efficiency is frequently displayed as mi/kWh. This is an economy metric where higher numbers denote superior efficiency.

To convert between metric kWh/100 km and British mi/kWh:

$$\text{mi/kWh} = \frac{100}{1.609344 \times (\text{kWh/100 km})} \approx \frac{62.137119}{\text{kWh/100 km}}$$

$$\text{kWh/100 km} = \frac{62.137119}{\text{mi/kWh}}$$

Standardized testing cycles and metric comparability

Converting numerical units through exact mathematical formulas reconciles arithmetic definitions, but it does not reconcile differences between underlying laboratory driving tests.

A vehicle tested under the European Worldwide Harmonized Light Vehicles Test Procedure (WLTP) follows different speed traces, acceleration phases, and ambient temperature requirements than a vehicle tested under the United States EPA 5-cycle procedure. Converting a European WLTP rating from L/100 km into mpg (US) yields an arithmetic equivalent for that specific test, but the resulting figure cannot be directly equated with an EPA window label rating. When evaluating vehicles, always verify both the unit and the underlying test cycle.

Practical steps for calculating fuel and energy costs

To calculate true travel expenses across different metrics, follow these steps:

1. Identify the unit type: Confirm whether the rating represents consumption (volume per distance) or economy (distance per volume).
2. Standardize the gallon: When evaluating miles per gallon, ensure you know whether the rating reflects US gallons (3.785411784 L) or imperial gallons (4.54609 L).
3. Calculate linear consumption: For financial projections, convert economy metrics into consumption metrics (L/100 km or kWh/100 km) to maintain linear financial proportionality.
4. Multiply by local tariffs: Multiply consumption by local fuel or electricity prices per unit to determine accurate cost per 100 kilometers or miles.

Understanding these formulas removes confusion, prevents calculation errors, and enables transparent comparisons across all automotive powertrains.

Key takeaways

• Efficiency metrics divide into distance-per-volume economy (mpg) and volume-per-distance consumption (L/100 km and kWh/100 km).
• The MPG illusion shows that gaining 5 MPG from 10 to 15 MPG saves about 33 gallons over 1,000 miles, whereas gaining 5 MPG from 30 to 35 MPG saves only about 5 gallons.
• International standards define 1 US gallon as 3.785411784 L, 1 imperial gallon as 4.54609 L, and 1 mile as 1.609344 km.
• Converting between mpg (US) and L/100 km uses the exact constant 235.214583, while converting between mpg (UK) and L/100 km uses 282.480936.
• Electric efficiency in kWh/100 km converts to mi/kWh by dividing 62.137119 by the kWh/100 km rating, providing consistent comparisons across borders.