Congressman Darrell Issa (R-CA) sent letters to Environmental Protection Agency and Department of Transportation officials October 18 asking them to provide legal argument as to why regulation of mobile source greenhouse gas emissions is different from regulating fuel economy through Corporate Average Fuel Economy standards. The letters, addressed to EPA’s air Administrator Gina McCarthy, transportation chief Margo Oge, and National Highway Traffic Safety Administrator David Stickland, were sent in response to testimony they gave at an oversight hearing the previous week.