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Monday, May 14, 2007

Should revised mpg mean a revised CAFE standard?

The Corporate Average Fuel Economy (CAFE) standard for American vehicles has required that each car manufacturer reach an average fuel economy standard for their fleet of vehicles: 27.5 mpg for cars, 22.2 mpg for light trucks. However, you may have noticed recent news about the EPA having overestimated - for some cars, drastically - the average fuel economy of the nation's vehicles.

Apparently, there are three different fuel economy figures. From the National Highway Transportation Safety Administration (NHTSA):
[There are] three different sets of fuel economy values- NHTSA’s CAFE values, EPA’s unadjusted dynamometer values, and EPA’s adjusted on-road values:
  • NHTSA’s CAFE values are used to determine manufacturers’ compliance with the applicable average fuel economy standards
  • The EPA's unadjusted dynamometer values are calculated from the emissions generated during the testing using a carbon balance equation. EPA knows the amount of carbon in the fuel, so by measuring the carbon compounds expelled in the exhaust they can calculate the fuel economy.
  • EPA’s adjusted on-road values are those values listed in the Fuel Economy Guide and on new vehicle labels, adjusted to account for the in-use shortfall of EPA dynamometer test values. (formatting mine)
The NHTSA values are the law for the manufacturer, but the manufacturer's fuel economy average is computed via one of two methods:
EPA is responsible for calculating the average fuel economy for each manufacturer. CAFE certification is done either one of two ways: 1) The manufacturer provides its own fuel economy test data, or 2) the EPA will obtain a vehicle and test it...using the same laboratory test that they use to measure exhaust emissions (fuel economy standard #2 from above).
This laboratory test has always been modified down to better mirror real world fuel economy (#3), so the CAFE standard has apparently been overestimating actual average fuel economy of the American fleet for years.

Bring that story up to date with this fact: the EPA estimate of on-road fuel economy has also been overestimating mpg for American cars, by as much as 50% (link discusses hybrids, but the overestimate applies to all vehicles). So, to summarize:

Actual fuel economy <>

If conserving fuel means saving fuel in the tank, not just on paper, then we need a CAFE test and a CAFE standard based on reality.

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