Postmaster General Louis DeJoy, defending the selection of a gasoline-powered vehicle maker to build the next generation of U.S. mail trucks, said he envisions the service moving quickly to electric power.

Oshkosh Corp. executives cautioned investors that they lacked expertise in building electric vehicles several months before the company won a lucrative U.S. Postal Service contract to build next-generation mail-delivery vans with batteries or internal-combustion engines.