PALO ALTO, Calif.—Auto makers have been laying the groundwork to seek relief from U.S. regulators on lofty fuel-economy targets. The Silicon Valley electric-car maker is preparing to make a public case this week for leaving mileage and emissions regulations intact, or making them even more stringent, a Tesla executive said. The company also will fight to keep other auto makers from loosening regulations in California, which has more ambitious targets than the federal government.