A Californian outfit, Energy CS, in Monrovia, CA has converted two Priuses to get up to 276mpg on lithium ion [ Saphion which are safer ] batteries. … He was inspired to alter his car, he said, after learning that Asian Prius models had a “stealth” button enabling them to be switched to electric-only mode until they hit a certain speed. The electronic tweaks he performed “fool the hybrid system into thinking the battery is fully charged” so it uses battery power at all speeds, rather than just during deceleration.” – telegraph
Some bloggers have said the numbers don’t add up. I’m getting a consistent 42 MPG in one without trying anything special.