EPA CAFE
[edit] Calculation
Fleet fuel economy is calculated using a harmonic mean, not a simple arithmetic mean (average).[3] The harmonic mean is the reciprocal of the average of the reciprocals of the fuel economies of the vehicles in the fleet. For a fleet composed of four different kinds of vehicles A, B, C, and D, produced in numbers nA, nB, nC, nD with fuel economies fA, fB, fC, fD the CAFE would be:
For example, a fleet of 4 vehicles getting 15, 13, 17, and 100 mpg has a CAFE of slightly less than 19 mpg:
The harmonic mean captures the fuel economy of the fleet for driving each car in the fleet for 1 mile while the arithmetic mean captures the fuel economy of the fleet for driving each car until one gallon of gas is burned (i.e. the 13 MPG vehicle would be driven for 13 miles while the 100 MPG vehicle would be driven for 100 miles).
For the purposes of CAFE, a manufacturer's car output is divided into a domestic fleet (vehicles with more than 75 percent U.S., Canadian or post-the North American Free Trade Agreement (NAFTA) Mexican content) and a foreign fleet (everything else). Each of these fleets must separately meet the requirements. The two-fleet requirement was developed by the United Automobile Workers (UAW) as a means to ensure job creation in the US. The UAW successfully lobbied Congress to write this provision into the enabling legislation. The UAW continues to advocate this position.[18] The two fleet rule for light trucks was removed in 1996.
Fuel economy calculation for alternative fuel vehicles multiplies the actual fuel used by a "Fuel Content" Factor of 0.15[19] as an incentive to develop alternative fuel vehicles.[20] Dual-fuel vehicles, such as E85 capable models, are taken as the average of this alternative fuel rating and its gasoline rate. Thus a dual-fuel E85 capable vehicle which gets 15 mpg on E-85 and 25 mpg on gasoline would be rated as 20 mpg for CAFE purposes, in spite of the fact that less than one percent of the fuel used in E85 capable vehicles is actually E85.[9]
Manufacturers are also allowed to earn CAFE "credits" in any year they exceed CAFE requirements, which they may use to offset deficiencies in other years. CAFE credits can be applied to the three years previous or three years subsequent to the year in which they are earned. The reason for this requirement is so that manufacturers are penalized only for persistent failure to meet the requirements, not for transient noncompliance due to market conditions.