The monthly values of the EDI are derived from two sub-indexes: (1) the average amount of fuel used per distance driven by newly purchased vehicles (EDIf), and (2) the distance driven per individual driver (EDId). The index and the two sub-indexes are computed monthly relative to their respective values in October 2007 (the nominal start of the 2008 model year—the first model year for which the EPA started using the current fuel-economy rating system). The EDI is computed by cross-multiplying EDIf and EDId. The lower the value of the EDI, the smaller the environmental impact.
EDIf estimates the relative amount of fuel needed to drive a fixed distance. EDIf is calculated as an inverse of the sales-weighted harmonic mean fuel economy of purchased new vehicles for each individual month.
EDId provides information about the relative amount of driving per licensed driver. EDId starts with the estimates of the total distance driven in the U.S. for each month as issued by FHWA. These raw distances are then adjusted by us to take into account (1) the seasonal variations in driving (in the U.S. more driving is done in the summer than in the winter), (2) the varying number of days in a month, (3) the continuously increasing number of drivers, and (4) the so-called rebound effect (increased amount of driving as a consequence of improved fuel economy of the new vehicle). (As recommended by the EPA, a rebound effect of 10% is included, meaning that 10% of the gain in fuel economy is assumed to be effectively lost due to increased amounts of driving.)
Page updated: Tuesday, January 22, 2013
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