Size or Mass? The Technical Rationale for Selecting Size as an Attribute for Vehicle Efficiency Standards

Vehicle efficiency standards defined by governments worldwide are indexed to either size or weight, setting less stringent targets for larger or heavier vehicles so as to reduce competitive impacts and promote efficiency improvements across an entire vehicle fleet. The choice of which of these two attributes to use has important consequences.

This white paper, the first in a series on best practices in fuel-economy and GHG-emissions standards design, summarizes the differences between size-based and mass-based standards and discusses their relative advantages and disadvantages. The authors argue that size-indexed standards promote efficient technologies more effectively, do not reward larger engines and diesels with artificially less stringent standards, and are less subject to gaming. These advantages argue for a fundamental reconsideration of the mass-based regulatory framework where it is in use and against its extension into as-yet-unregulated light-duty vehicle markets.

As governments in major vehicle markets define new standards for 2020 and beyond, that reconsideration is an urgent priority. The development of lightweight materials, such as ultra-high-strength steel, aluminum, plastics, magnesium, and carbon fiber, is progressing rapidly. Continued use of mass-based standards will discourage the deployment of these lightweight materials and result in a missed opportunity to significantly reduce fuel consumption and carbon emissions worldwide.

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