Tank-to-Wheel (TTW)

Introduction

The term "tank-to-wheel" (TTW) refers to the emissions generated during a vehicle’s actual operation. It encompasses only direct exhaust emissions —namely CO₂, nitrogen oxides, or particulate matter—resulting from fuel combustion or energy use in the powertrain. Unlike the well-to-wheel (WTW) approach or life cycle assessment (LCA), TTW does not consider upstream emissions from the extraction, processing, or transport of energy sources.

Thus, TTW constitutes an important but limited component of the overall assessment of a vehicle’s climate impact.

Significance of the TTW Approach

The TTW approach is a key component when it comes to measuring direct vehicle emissions. It is limited to what actually occurs at the exhaust or in the powertrain during operation—that is, to the emissions generated directly while driving.

TTW is particularly relevant in two areas:

• Regulatory emissions testing: Many legal limits, such as the EU fleet limits for passenger cars, are based exclusively on real-world driving data. This makes them easy to compare, but it excludes upstream emissions.
• Technology comparison in operation: While electric vehicles are considered zero-emission under the TTW approach, internal combustion engine vehicles generate emissions directly while driving.
  

This makes TTW a simple, measurable, and standardizable criterion for policymakers and businesses—though it is only one part of the bigger picture.

Use Cases and Examples

In practice, TTW is frequently used to compare and report vehicle-level emissions.

Typical applications include:

• Comparison of Different Drive Systems:
  • Electric vehicles = 0 g/km TWP emissions (zero local emissions).
  • Diesel or gasoline vehicles = direct CO₂ emissions during operation.
  • Hydrogen-powered vehicles = depending on the technology, they are also virtually emission-free in the TTW sector.
• Fleet management: Companies that electrify their fleets can directly reduce their Scope 1 emissions, as TTW emissions are directly included in the company’s balance sheet.
• Communication: Manufacturers use TTW data to market vehicles as “locally emission-free.”
  

A clear example: A battery-electric vehicle produces no direct emissions during its in-use phase. Nevertheless, its life-cycle assessment can be significantly impacted by the electricity mix and battery production—a limitation that must be taken into account when interpreting in-use results.

Challenges and Limitations

Although TTW has many advantages, the approach faces clear methodological limitations.

• No consideration of upstream processes: Emissions from electricity generation, refineries, or the transport of energy sources are not included.
• Risk of distortion: Vehicles may appear “clean” in a TTW comparison, even though their overall performance is worse under the WTW or LCA approach.
• Limited validity: TTW alone is not sufficient for climate strategies at the corporate or policy level—supplementary analyses are necessary.
  

To ensure that TTW data is used correctly, it should always be viewed in the context of other valuation approaches.

Integration with Standards & Reporting

The TTW approach is incorporated into various standards, but plays a specific role in each one:

• GHG Protocol: TTW emissions are generally classified as Scope 1, as they are generated directly within the company's own operations.
• Relation to WTW: Only when combined with Well-to-Tank (WTT) does a complete picture emerge (WTW).
• CSRD / ESRS: Companies with vehicle fleets must report their TTW emissions. However, this is not sufficient to provide a comprehensive picture of their climate impact—a WTW analysis is required here.

Practical Tips for Businesses

To effectively integrate TTW into your own climate strategy, there are a few practical steps you can take:

• Systematic data collection: e.g., via fuel cards, trip data, or charging logs.
• Transparency in communication: Make it clear that TTW represents only a portion of the overall balance sheet.
• Integration into strategies: Use TTW as a short-term lever (e.g., through e-mobility), but always combine it with TEA or LCA analyses.

Further reading

• Agora Verkehrswende: Carbon Footprint of Battery-Powered and Alternative Drivetrains (PDF)
• GHG Protocol – Scope 3 Draft 2024