Diesel is still king for longer journeys, although the best (non-plug-in) hybrids are catching up now. A Volkswagen Golf 2.0 TDI diesel will pretty easily do 60mpg on a gently-driven long-haul. If diesel prickles your conscience, then consider a Toyota Corolla hybrid — now that the Prius is no longer on sale, this is Toyota’s long-haul economy champion, and the updated 140hp hybrid should see you get close to 60mpg on long runs.