Toyota Prius owners tend to be a proud lot since they drive the fuel-efficient hybrid gas-electric car that's ... one of the hottest-selling vehicles in America. A few, however, felt that good was not good enough. They've made improvements" even though the modifications voided parts of their warranties. Why? Five words: one hundred miles per gallon. "We took the hybrid car to its logical conclusion," [Felix] Kramer says, by adding more batteries and the ability to recharge by plugging into a regular electrical socket at night. Compared with the Prius' fuel efficiency of 50 mpg, plug-in hybrids use half as much gasoline by running more on cleaner, cheaper, domestic electricity. These trendsetters monkeyed with the car ... to make a point: If they could make a plug-in hybrid, the major car companies could, too. Kramer ... and a cadre of volunteers formed the California Cars Initiative (online at calcars.org). They added inexpensive lead-acid batteries ... giving the car over 100 mpg in local driving and 50 to 80 mpg on the highway. The cost of conversion is about $5,000 for a do-it-yourselfer. Several small companies like EnergyCS ... started doing small numbers of conversions for fleets and government agencies using longer-lasting, more energy-dense lithium-ion batteries. Kramer hired EnergyCS to convert his Prius and reported on a typical day of driving. Compared with driving his Prius before the conversion, he ... spewed out two-thirds less greenhouse gases at a total cost of $1.76 for electricity and gasoline, instead of the $3.17 it would have required on gasoline alone. People want plug-in hybrids but can't get them. Dealers don't sell them yet, and the few conversion services cater to fleets.