The effect of fuel economy on fuel cycle emissions was considered for Cases C and D. Case D is based on high efficiency vehicle fuel economy. The net effect of fuel economy improvements in Case D is a 33 percent reduction in fuel cycle emissions. This outcome depends to some extent on the trend in fuel tank size as vehicle fuel economy is improved. If fuel tank size were reduced in proportion to improvements in fuel economy, a 33 percent improvement in fuel economy would result in a somewhat smaller improvement in fuel cycle emissions.
The potential diesel, LPG, methanol, and electric vehicles to occupy different portions of the light-duty vehicle market were considered in Case C. The fuel economy calculations shown in Section 5 identify the magnitude of this effect. The shift in vehicle fuel economy due to expected vehicle market share is about 10 percent. For example, LPG vehicles are expected to displace larger gasoline vehicles with an average fuel economy of 19 mpg. This result indicates that the benefit of displacing gasoline vehicles would be greater that that of an electric vehicle. Similarly, the fuel cycle emissions from the LPG vehicles would be greater due to their larger size.