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Originally Posted by camaro-dreamer
The government has prepared a report summarizing CAFE for you. http://www.nhtsa.gov/staticfiles/rul...al_Rule_FR.pdf
A couple of things are important. First, CAFE uses a harmonic mean. This means that vehicles which produce more CO2 count very heavily against manufacturers. However, CAFE also takes into account the footprint of vehicles. A formula for vehicle footprint is found in the above link.
Small vehicles with gas guzzling engines are very bad for manufacturers. Bigger vehicles, by virtue of their larger footprint, count less. To a certain extent, manufacturers can play the system to produce larger footprint cars and get a more lenient fuel economy standard. At the same time, these larger vehicles weigh more thus reducing fuel economy. There must be a fine balancing act.
Mathematically, there is a formula for determination of a fuel economy target in a given year. The formula can be found in the link I posted above. The parameters in the equation are given a physical interpretation. Their selection seems arbitrary though. |
Its nice to see someone else who has read up on this stuff. One thing to add to that is that the fuel economy rating that we see on the window sticker isn't the same as the fuel economy used in the CAFE. The CAFE rating is based on a much older version of the EPA's fuel economy test and is around 20% higher than what we normally see. So if a car is advertised as being 19 city, 29 highway, 24 combined its CAFE score is going to be up around 29 mpg.
Also, to further add on to what you posted, the harmonic mean increases the importance of vehicles with low efficiency compared to a simple average. So it ends up that it is more important for an automaker to get a 2 mpg improvement on something that gets say 20 mpg than it is to get a 4 mpg gain on a car already getting 40 mpg. I think this is going to result in a lot more diesels in trucks and SUVs, particularly for the Detroit automakers who are very truck heavy compared to the imports.