Metrics & measurement methods

Decision makers should determine how critical metrics within their region's SSR will be measured. These choices have significant implications for the real-world impact of their regulation. This section includes:

• A comparison of alternative methods for measuring drive cycles, and understanding how these choices affect the accuracy of real-world emissions estimates.
• An evaluation of whether and how to include upstream or full lifecycle emissions in your regulation to better capture the total environmental impact of vehicles.

Decision makers should use this information to select measurement approaches that align with their policy goals, whether they are prioritizing simplicity, accuracy, enforceability, or environmental ambition.

Measuring SSR metrics

SSRs typically rely on one of three primary indicators: (1) the amount of energy consumed by a vehicle, (2) the GHG emissions it releases into the atmosphere, and (3) its classification as a fully electric vehicle. In this technical note, we will first describe the common units of measurement and their implications, and then explain the standardized testing procedures used to calculate these indicators.

Key performance metrics for vehicles

Measuring vehicle energy consumption

Vehicle energy consumption is typically expressed in one of two ways¹:

• Energy Efficiency: This metric refers to the distance a vehicle can travel per unit of energy consumed (e.g., miles per gallon or kilometers per liter). A vehicle's energy efficiency increases when a given distance can be traveled with a reduced amount of energy input.
• Energy Intensity: This is the inverse of energy efficiency, representing the amount of energy consumed per unit of distance (e.g., gallons per 100 miles or liters per 100 kilometers). Using less energy to travel a certain distance reduces the vehicle's intensity.

One key distinction between these two measures is that energy intensity provides a linear scale with respect to fuel consumption, making it easier to demonstrate improvements. In contrast, energy efficiency is nonlinear in relation to fuel consumption, meaning that small increases in efficiency, especially for less efficient vehicles, can result in disproportionately larger reductions in fuel consumption. For this reason, energy intensity is often preferred by policymakers to quantify regulatory progress, as it offers a clearer and more straightforward way to track improvements. See Box 1 for more details².

Box 1. The MPG illusion

The Science paper "The MPG Illusion" (Larrick, 2008) highlights a common misconception about fuel efficiency when expressed in miles per gallon (MPG). People tend to assume that improvements in MPG result in a linear increase in fuel savings, but this is misleading. The relationship between MPG and fuel consumption is non-linear, meaning small increases in MPG for less fuel-efficient vehicles can lead to significant savings, while similar improvements in already fuel-efficient vehicles result in much smaller benefits. For example, increasing a car's efficiency from 10 to 15 MPG saves more fuel over a given distance than increasing from 30 to 35 MPG. The paper argues that expressing fuel efficiency in terms of gallons per mile (GPM) or liters per 100 kilometers (L/100 km), as is more common outside the US, provides a clearer picture of fuel savings and can help consumers make more informed decisions.

Measuring vehicle emissions

The measurement of GHG emissions, specifically CO₂ or CO₂ equivalent (CO₂e), is another critical metric for SSRs. These emissions are typically expressed in grams per mile (g/mi) or grams per kilometer (g/km). The emissions a vehicle produces per mile or kilometer depend on³:

• The fuel type: Diesel, for example, produces more emissions per liter compared to gasoline.
• The energy consumption of the vehicle: More fuel-efficient vehicles emit less CO₂ per mile or kilometer.

BEVs produce no tailpipe emissions, while FCEVs running on hydrogen emit only water vapor. However, determining tailpipe emissions for PHEVs, which use both electricity and gasoline, is more complex. When operating solely on electricity, PHEVs emit no tailpipe emissions, but when using gasoline, their emissions depend on fuel efficiency. As a result, a PHEV's overall tailpipe emissions vary based on factors like battery capacity, driving patterns, and charging frequency.

It's important to note that tailpipe emissions also include additional air pollutants (such as carbon monoxide, nitrogen oxides, and particulate matter). However, for the purposes of SSRs, these are often not included in the regulations, and thus, this note focuses solely on GHG emissions.

Measuring driving range for PEVs

For PEVs, in addition to energy efficiency and emissions, driving range is a key indicator. Driving range is primarily determined by the battery's capacity (measured in kWh) and its efficiency in converting stored energy into miles driven (measured in miles or kilometers per kWh). California's ZEV standard, for instance, establishes minimum driving range requirements for BEVs to qualify as compliant vehicles. Under current regulations, a BEV must achieve a minimum range of 50 miles to meet ZEV standards⁴. The forthcoming Advanced Clean Cars II (ACC II), effective for model year 2026 and beyond, will require a minimum certification range of at least 200 miles for a BEV to qualify as a ZEV⁵. This range is typically measured under specific test conditions which simulate real-world driving patterns to estimate how far an electric vehicle can travel on a full charge.

Testing methods for measuring efficiency, emissions, and driving range

Vehicle efficiency, emissions, and driving range for PEVs are typically measured under controlled laboratory conditions, with the aim of simulating real-world driving behaviors. A key tool in these assessments is the "drive cycle" – predefined driving patterns that replicate typical driving conditions, such as acceleration, deceleration, cruising, and idling. Drive cycles are typically applied using a chassis dynamometer, a device that simulates road conditions by allowing the vehicle to operate while stationary. The vehicle's wheels are placed on rollers, and the dynamometer can simulate different driving environments, such as urban stop-and-go traffic or highway cruising, allowing for a controlled and repeatable testing process. These tests provide a standardized approach to measuring how vehicles perform under different driving conditions.

Commonly used drive cycles

Several widely used drive cycles, each tailored to different parameters and driving conditions, include the following examples:

• New European Driving Cycle (NEDC): Once the standard test in Europe, the NEDC was designed to simulate typical urban and suburban driving. However, it has been criticized for being overly optimistic and failing to account for real-world variables. As a result, it has largely been phased out in Europe.
• Worldwide Harmonized Light Vehicles Test Procedure (WLTP): Introduced in 2017 to replace the NEDC, the WLTP incorporates more stringent testing conditions to better reflect real-world driving. The 2015 Dieselgate scandal revealed the extent to which manufacturers could exploit weaknesses in testing procedures to manipulate results, most notably through strategies that optimized vehicle performance specifically for the test environment⁶. The WLTP was established to close these loopholes by introducing stricter controls over testing conditions, including adjustments for tire pressure, vehicle mass, ambient temperature, and more dynamic driving patterns that better reflect real-world conditions^7,8,9. The WLTP cycle was developed using real-driving data collected globally, making it more representative of diverse driving conditions. It is divided into four parts based on different average speeds: low, medium, high, and extra high, each featuring various driving phases such as stops, acceleration, and braking. Additionally, each powertrain configuration is tested in both its lightest (most economical) and heaviest (least economical) versions to capture a broader range of vehicle performance.
• EPA Federal Test Procedure (FTP): In the US, the Environmental Protection Agency (EPA) employs several drive cycles to evaluate vehicle performance under different driving conditions. The Urban Dynamometer Driving Schedule (UDDS) is designed to simulate stop-and-go city traffic, characterized by frequent acceleration and deceleration, along with idling periods. The Highway Fuel Economy Test (HWFET), on the other hand, simulates steady highway driving, where vehicles maintain relatively constant speeds¹⁰. Figure 1 illustrates the UDDS and HWFET drive cycle schemes.
• Japanese JC08 Cycle: Designed to simulate driving conditions typical of congested urban traffic in Japan for light vehicles weighing under 3,500 kg, the JC08 test includes idling periods and frequent acceleration and deceleration. Emissions are measured under two conditions: A cold start and a warm start, with the test being utilized for both emission assessments and fuel economy evaluations for gasoline and diesel engine vehicles. The JC08 cycle was fully phased in by October 2011, incorporating a weighted average of different cycles for emission calculations. Over the years, the proportions of the JC08 mode in the cold and hot starts were adjusted, culminating in the final parameters that include a duration of 1,204 seconds, a total distance of 8.171 km, an average speed of 24.4 km/h (34.8 km/h excluding idling), a maximum speed of 81.6 km/h, and a load ratio of 29.7%¹¹.

Figure 1. Schematic representation of the UDDS and HWFET drive cycles

Real Driving Emissions (RDE)

Despite the advancements in laboratory testing methods, challenges remain in accurately simulating real-world driving conditions. The results obtained from laboratory tests, while refined, still provide only an approximation of actual vehicle performance on the road. To bridge this gap, additional testing methods have been developed, allowing for a direct comparison between laboratory-based results and actual on-road performance.

A prominent example is the Real Driving Emissions (RDE) test procedure^12,13,14. Since September 2017, the RDE testing protocol has been a mandatory stage in the type-approval process for new passenger cars and light commercial vehicles in the European Union. The RDE test is designed to complement the WLTP laboratory tests by evaluating emissions under actual driving conditions. This on-road test measures pollutants emitted during typical use, capturing variations in driving patterns, speeds, and environmental conditions. By using Portable Emissions Measurement Systems (PEMS), RDE ensures that vehicles comply with emission limits in real-world scenarios, promoting more accurate and reliable assessments of a vehicle's environmental impact. For example, a vehicle must meet specific nitrogen oxides (NO_x) limits during the RDE test, ensuring that its emissions remain below a designated threshold¹⁵. This requirement encourages manufacturers to design vehicles that not only perform well in controlled tests but also adhere to strict emission standards during everyday driving.

Notes to the policymaker

As drive cycles and testing methodologies improve, policymakers are encouraged to adopt the most recent standards, such as the WLTP, when designing SSRs. However, the implementation of these modern standards must be balanced with an understanding of local conditions and existing regulatory frameworks. Policymakers should evaluate the strengths and limitations of various testing methods to ensure that the chosen approach aligns with both current standards and regional circumstances. For countries with established regulations based on earlier testing methods, such as the NEDC, it may be more pragmatic to continue using those methods, supplemented by conversion factors to achieve consistency with newer standards (For further details on conversion factors, please consult Table E.1 in the European Commission Report (p. 3)¹⁶. In this context, it is essential to recognize that transitioning from less stringent procedures, like the NEDC, to more rigorous standards, such as the WLTP, may result in higher recorded emission values (Figure 2). Ultimately, by carefully selecting and implementing appropriate testing methodologies, policymakers can better ensure that real-world vehicle performance is accurately reflected.

Figure 2. Average CO₂ emissions under NEDC versus WLTP for the European Union standard¹⁷.

Measuring upstream & lifecycle emissions

The EU currently measures CO₂ emissions from light-duty vehicles using the WLTP —in place since 2017. The WLTP reflects real-world driving conditions, providing accurate CO₂ emission and fuel consumption values. For each new passenger car, specific CO₂ emissions are calculated based on the vehicle's mass based on the following formula:

${\mathrm{Specific\; emissions\; of\; CO}}_2=95+a\cdot (M-M_0)$

Where:

$M=\mathrm{Mass\; in\; running\; order\; of\; the\; vehicle\; in\; kilograms\; (kg)}$

$M_0=\mathrm{1,379.88}$

$a=0.0333$

The WLTP and associated EU CO₂ standards represent a state-of-the-art approach to account for tank-to-wheel emissions, measured directly from the vehicle's tailpipe. However, this framework faces limitations when evaluating the environmental impacts of electric vehicles (EVs), which do not produce tailpipe emissions but instead shift them upstream to power plants where the electricity used for their operation is generated.

Notably, under the WLTP—and similarly under the U.S. drive cycle protocol—EVs are reported as emitting 0 gCO₂/mi, regardless of their size. While this policy has significantly fostered EV adoption and production, it has also led to unintended consequences. Larger EVs receive disproportionately more credits because the higher emissions allowances for larger vehicle categories amplify the emissions differential. As a result, OEMs, aiming to maximize profitability, have been incentivized to produce larger, less energy-efficient EVs. This trend undermines the intended gains in energy efficiency and emissions reductions. Figure 3 illustrates this effect.

Figure 3. Mechanics of emissions credit allocation, highlighting the disproportionate credit advantage for larger EVs.

This accounting system enables EV manufacturers to earn emissions credits that may not fully align with the actual emissions reductions achieved by their vehicles. These credits can subsequently be sold to higher-polluting manufacturers, allowing them to meet regulatory targets while potentially limiting tangible emissions reductions. This dynamic may reduce the overall effectiveness of the regulatory framework in driving innovation and achieving meaningful progress toward emissions reductions.

Addressing upstream emissions associated with EV operations could help mitigate these challenges. However, as electricity grids decarbonize globally, a growing share of emissions related to EVs originates from other stages of their lifecycle—such as material extraction, battery production, and disposal—rather than from vehicle operation.

Life Cycle Assessment (LCA) provides a comprehensive solution by evaluating the environmental impacts of a product across all phases of its life, including raw material extraction, manufacturing, use, and disposal. By capturing emissions from non-operational phases, such as production and end-of-life management, LCA ensures a complete understanding of a product's environmental footprint. Incorporating LCA into CO₂ emissions regulations enables a more accurate representation of the true environmental costs and benefits of EVs compared to conventional vehicles.

Recognizing this need for a more holistic approach, the European Union adopted an amendment to the EU Light Duty Vehicle CO₂ standards on March 28, 2023. The amendment introduced a 100% CO₂ emission reduction target for all new cars registered from 2035 onwards, strengthening the 2030 55% CO₂ reduction target for cars, relative to a 2021 baseline. Critically, the new regulation introduces a life-cycle CO₂ emissions assessment in the CO₂ standards. The European Commission has instructed a committee to develop a life cycle assessment methodology by the end of 2025. Manufacturers will be able to apply this methodology starting in January 2026 to voluntarily report life-cycle CO₂ emissions. This information will be considered in bi-annual status reports for analyzing the progress toward climate neutrality by 2050.

Table 1 & Table 2 present methodologies that account for upstream and life cycle emissions, respectively.

Table 1. Tools and Methodologies to account for upstream emissions

Table 2. Tools and methodologies to account for life cycle emissions

Footnotes

¹ U.S. Department of Energy. (2025). Energy efficiency vs. energy intensity. Office of Energy Efficiency & Renewable Energy. Retrieved February 24, 2025, from https://www.energy.gov/eere/analysis/energy-efficiency-vs-energy-intensity

² Larrick, R. P., & Soll, J. B. (2008). The MPG Illusion. Science, 320(5883), 1593–1594. https://doi.org/10.1126/science.1154983

³ EPA (2025). Greenhouse gas emissions from a typical passenger vehicle. Retrieved February 24, 2025, from https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-typical-passenger-vehicle

⁴ California Code of Regulations, Title 13, § 1962.2. (2022). Zero-Emission Vehicle Standards for 2018 Through 2025 Model Year Passenger Cars, Light-Duty Trucks, and Medium-Duty Vehicles. Retrieved from https://govt.westlaw.com/calregs/Document/I830D31267AE611EDB6DFBD43FBB6EAB8?viewType=FullText&originationContext=documenttoc&transitionType=CategoryPageItem&contextData=(sc.Default)&bhcp=1

⁵ California Code of Regulations, Title 13, § 1962.4. (2022). Zero-Emission Vehicle Requirements for 2026 and Subsequent Model Year Passenger Cars and Light-Duty Trucks. Retrieved from https://govt.westlaw.com/calregs/Document/IB66C9D507AEE11ED90EF9C5CC5AED63A?viewType=FullText&originationContext=documenttoc&transitionType=CategoryPageItem&contextData=(sc.Default)

⁶ Topham, G. (2015). Volkswagen emissions scandal: No fix before end of year. BBC News. https://www.bbc.com/news/business-34324772

⁷ European Commission. (2017). Commission Implementing Regulation (EU) 2017/1153 of 2 June 2017 setting out a methodology for determining the correlation parameters necessary for reflecting the change in the regulatory test procedure on the CO₂ emissions and fuel consumption of light-duty vehicles and amending Regulation (EU) No 1014/2010. Official Journal of the European Union. https://eur-lex.europa.eu/eli/reg_impl/2017/1153/oj

⁸ European Automobile Manufacturers Association. (2025). Worldwide Harmonised Light Vehicle Test Procedure (WLTP). Retrieved February 24, 2025, from https://www.wltpfacts.eu/

⁹ Vehicle Certification Agency. (2021). The Worldwide Harmonised Light Vehicle Test Procedure (WLTP). https://www.vehicle-certification-agency.gov.uk/fuel-consumption-co2/the-worldwide-harmonised-light-vehicle-test-procedure/

¹⁰ EPA (2025). Dynamometer drive schedules. Retrieved February 24, 2025, from https://www.epa.gov/vehicle-and-fuel-emissions-testing/dynamometer-drive-schedules#FTP

¹¹ Japan Inspection Organization. (2024). Worldwide Harmonized Light Vehicles Test Cycle (WLTC) in Japan. Retrieved February 24, 2025, from https://japaninspection.org/worldwide-harmonized-light-vehicles-test-cycle-wltc-in-japan/

¹² European Commission. (2019). Questions and answers – Real driving emissions tests for cars. https://ec.europa.eu/commission/presscorner/detail/en/qanda_19_2850

¹³ ICCT. (2018). Changes to the motor vehicle type-approval system in the European Union. International Council on Clean Transportation. https://theicct.org/sites/default/files/publications/EU-Type-Approval-System_ICCT-Policy-Update_20180529_vF.pdf

¹⁴ Car Emissions Testing Facts. (2025). RDE: What is the real driving emissions test? Retrieved February 24, 2025, from https://www.caremissionstestingfacts.eu/rde-real-driving-emissions-test/

¹⁵ ICCT. (2017). Real-driving emissions test procedure for exhaust gas pollutant emissions of cars and light commercial vehicles in Europe. International Council on Clean Transportation. https://theicct.org/sites/default/files/publications/EU-RDE_policy-update_18012017_vF.pdf

¹⁶ Tsiakmakis, S., Fontaras, G., Cubito, C., Pavlovic, J., Anagnostopoulos, K., & Ciuffo, B. (2017). From NEDC to WLTP: Effect on the type-approval CO₂ emissions of light-duty vehicles (EUR 28724 EN). Publications Office of the European Union. https://publications.jrc.ec.europa.eu/repository/bitstream/JRC107662/kjna28724enn.pdf

¹⁷ European Environment Agency. (2024). CO₂ emissions performance of new passenger cars in Europe. https://www.eea.europa.eu/en/analysis/indicators/co2-performance-of-new-passenger