They drive themselves to the gas station. By the time self-driving cars come to market, Uber will have had plenty of time to negotiate partnerships with severa

They drive themselves to the gas station. By the time self-driving cars come to market, Uber will have had plenty of time to negotiate partnerships with several gas station companies, whose full-service lane attendants will use some sort of scanner/app to verify the car for charge purposes, and Uber will probably cross-verify gas tank levels automatically to make sure they're not being scammed by stations (eg 10 gallons for you, 1 for me).