Compare vehicle fuel and electricity consumption ratings

The short answer

Two consumption ratings can be compared fairly only when three things line up. First, each rating must belong to the exact vehicle configuration you are considering: model year, market, powertrain, drivetrain, trim and wheels. Second, both must be expressed in the same unit, which simple arithmetic can fix. Third, both must come from the same test system, which arithmetic cannot fix. A US EPA rating and a European WLTP rating can be converted into matching units, but that does not make them equivalent, because the underlying tests differ. Ratings also measure consumption only. They do not tell you what a vehicle will cost to run or how far it will actually go on a tank or a charge.

What each unit measures

Consumption figures come in two opposite forms. Some state distance per amount of fuel, where a higher number is better. Others state energy or fuel used per distance, where a lower number is better. Mixing them up is the most common comparison error.

mpg (miles per gallon): distance traveled per volume of fuel. Higher is better. A US gallon is 3.785412 liters and an Imperial (UK) gallon is 4.54609 liters, so always confirm which gallon a figure uses.
L/100 km (liters per 100 kilometers): fuel volume used per fixed distance. Lower is better.
kWh/100 km or kWh/100 miles: electrical energy used per fixed distance. Lower is better. One kilowatt-hour (kWh) equals 3.6 megajoules.
MPGe (miles per gallon equivalent): a US measure for electric vehicles that treats 33.7 kWh as equal to the energy in one US gallon of gasoline. It is distance per equivalent energy, not battery range and not operating cost.

Converting between units

These formulas use exact factors of 1.609344 km per mile, 3.785412 liters per US gallon and 4.54609 liters per Imperial gallon. The same formula converts in both directions because the relationship is reciprocal.

L/100 km      = 235.215 / US mpg
US mpg        = 235.215 / (L/100 km)
L/100 km      = 282.481 / Imperial mpg

kWh/100 miles = kWh/100 km x 1.609344
kWh/100 km    = kWh/100 miles / 1.609344
MPGe          = 3370 / (kWh per 100 miles)

Worked example (fuel): a 50 mpg US combined rating converts to 235.215 / 50 = 4.70 L/100 km. The figure is still an EPA combined rating. Only the unit has changed, and it is not a prediction of real-world consumption.

Worked example (electricity, hypothetical): an EV rated at 20 kWh/100 km uses 20 x 1.609344 = about 32.19 kWh/100 miles. Applying the 33.7 kWh-per-gallon equivalence gives 3370 / 32.18688 = about 104.7 miles per gasoline-equivalent gallon. This is arithmetic only. It does not create an EPA-certified MPGe rating, because the original 20 kWh/100 km figure came from a different test.

Why mpg differences can mislead

Because mpg is distance per gallon, equal mpg gains do not save equal amounts of fuel. Going from 20 to 25 mpg cuts fuel use from 5 to 4 gallons per 100 miles, saving 1 gallon. Going from 40 to 50 mpg cuts it from 2.5 to 2 gallons per 100 miles, saving only half a gallon. When you compare vehicles on fuel used rather than miles gained, convert to gallons per 100 miles (100 / mpg) or to L/100 km. A per-distance figure also makes later cost estimates straightforward.

Match the exact configuration first

A model name alone does not identify a rating. Toyota's 2023 US brochure for the Corolla Hybrid shows how much the figure moves within one nameplate. The LE front-wheel-drive version has a 1.8-liter four-cylinder hybrid powertrain rated at 138 hp, an electronically controlled continuously variable transmission (ECVT) and standard 16-inch steel wheels. Toyota lists separate EPA estimates for other trims and drivetrains:

2023 Corolla Hybrid (US)	EPA city / highway / combined (US mpg)	Combined converted to L/100 km
LE FWD	53 / 46 / 50	4.70
LE AWD	51 / 44 / 48	4.90
SE FWD	50 / 43 / 47	5.00
SE AWD	47 / 41 / 44	5.35

A used-car listing that says only "Corolla Hybrid" could therefore carry a combined rating anywhere from 44 to 50 mpg. Before you choose a figure, confirm the trim and drivetrain from the vehicle documents or badging.

Electric vehicles vary in the same way. Polestar's 2023 owner's manual for the Polestar 2 sold in Ireland gives WLTP consumption as ranges for each version. Those ranges reflect different equipment, so a range alone does not identify a particular car's certified value:

2023 Polestar 2 (Ireland, WLTP)	kWh/100 km	Converted to kWh/100 miles
Standard range Single motor	16.5 to 17.8	26.55 to 28.65
Long range Single motor	17.0 to 18.4	27.36 to 29.61
Long range Dual motor	19.3 to 20.2	31.06 to 32.51

Check the test system: EPA vs WLTP

Consumption ratings come from laboratory tests that follow a set speed-over-time schedule. The US EPA reports city, highway and combined figures. Since the 2008 model year, those figures have been adjusted using additional tests for higher speeds, air-conditioning use and cold temperatures. WLTP (Worldwide Harmonised Light Vehicle Test Procedure), used in Europe and other markets, averages urban, suburban, extra-urban and motorway phases into its own figure.

Because the procedures differ, converting a WLTP kWh/100 km value into MPGe, or an EPA mpg value into L/100 km, matches the units but not the test basis. In the examples above, the Corolla figures are EPA ratings and the Polestar figures are WLTP ratings. They can sit side by side if each row is labeled with its system, but they do not support a ranking of which vehicle is more efficient. For a fair efficiency ranking, find both vehicles rated under the same system, ideally for the same market.

Comparing a gasoline car with an electric car

For US-market vehicles, the EPA's MPGe figure gives electric and gasoline vehicles a common energy basis, so an EV's MPGe can be set against a gasoline car's mpg when both are EPA ratings. You can also go the other way and express a gasoline rating as energy per distance. At 33.7 kWh per gallon, a 50 mpg car uses the energy equivalent of 33.7 / 50 x 100 = 67.4 kWh per 100 miles. That puts it on the same scale as an EV's kWh/100 miles figure, again only when both ratings come from the same test system.

Keep in mind what MPGe leaves out. It treats a gallon of gasoline and 33.7 kWh of electricity as the same energy, but the two cost different amounts and are bought in different ways. A higher MPGe does not by itself mean a lower running cost.

Keep consumption, cost and range separate

Consumption is the rated figure itself: mpg, L/100 km, kWh/100 km or MPGe. Use it to compare efficiency on a shared test basis.
Operating cost requires a per-distance consumption figure multiplied by your local fuel or electricity price. Prices change, so calculate cost as a separate step with current local rates rather than inferring it from a rating.
Certified range is a comparison value from the test, not the distance you should expect to drive. Polestar's manual notes that load, wheels, driving style, speed, weather and road conditions all affect real consumption and range.

Which rating to use for your comparison

Choose the approach that fits what you are comparing:

Comparing US-market vehicles, gasoline or electric: use EPA ratings for each exact configuration. Compare mpg with mpg, or MPGe with mpg, and convert to a per-distance unit if fuel use or cost matters more than miles per unit.
Comparing European or other WLTP-market vehicles: use WLTP figures for each exact version and equipment level, in kWh/100 km or L/100 km.
Comparing a US-rated vehicle with one rated only under WLTP: convert units if it helps you read the numbers, but label each figure with its test system and do not treat a small gap as a meaningful difference.
Comparing used-car listings: identify the trim, drivetrain, battery, wheels and model year before you assign any rating. A listing's headline figure may belong to a different version than the car being sold.

A worksheet for consistent comparisons

Recording the same fields for every vehicle keeps comparisons honest and shows at a glance whether two figures can be ranked. One row per vehicle with these columns works well:

Vehicle, model year and market
Engine or electric motor, transmission and battery
Drivetrain, trim, wheel size and relevant options
Source document and its date
Rating system (EPA or WLTP) and phase (city, highway, combined)
Original value and unit
Conversion formula used and converted value
Whether the rating matches this exact configuration (yes / no / unknown)
Certified range, kept in its own column
Estimated operating cost, kept in its own column and calculated separately
Comparison decision: rank, compare with caution, or do not rank

Applied to the earlier examples, the Corolla Hybrid LE FWD row would show EPA combined 50 mpg (4.70 L/100 km), configuration confirmed. A Polestar 2 Long range Dual motor row would show WLTP 19.3 to 20.2 kWh/100 km, with the individual car's certified value still to be confirmed from its equipment. The decision column would read "do not rank", because the systems differ. To rank the two, you would first need ratings for both under the same system.