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GREEN CITY Hybrid cars — those that run on a combination of gasoline and electricity — are all the rage among drivers looking to go green. But imagine a car that could drive 100 miles on one gallon of gas. That's what a hybrid could get if converted into a plug-in version, something Bay Area residents are starting to do themselves, filling a void left by the auto industry.

The California Car Initiative (a.k.a. CalCars) is on a mission to make plug-in hybrid electric vehicles widely available. In collaboration with organizations like Plug-In Partners and Plug-In Bay Area, CalCars is on a mission to persuade carmakers to mass-produce plug-in hybrid vehicles. The technology already exists, allowing our cars to be much more fuel efficient.

The first prototype PHEV was created by CalCars in 2004. This Palo Alto nonprofit converted a Toyota Prius into a Prius+, a plug-in hybrid able to travel more than 100 miles using only one gallon of fuel.

A PHEV is essentially a hybrid that has additional battery capacity and can be recharged from a household 120-volt electrical outlet. CalCars promotional materials explain the way a plug-in hybrid works: "It's like having a second fuel tank that you always use first — only you fill up at home, from a regular outlet, at an equivalent cost of under $1 per gallon."

"Conversions are a strategy, not an end in themselves," Felix Kramer, CalCars founder, told the Guardian. "The game is all about getting hundreds of thousands of PHEVs on the road from carmakers."

Toyota recently announced it will be testing PHEV prototypes this fall in Japan, Europe, and the United States. General Motors has also announced it is working on a plug-in hybrid called the Volt, to be publicly released in 2010. A handful of other car companies have expressed their intention to produce PHEVs but haven't given release dates.

Public support by municipalities — including San Francisco, which passed a resolution to support PHEVs in 2006 — is also putting pressure on car manufacturers. Until plug-in hybrids are put on the market, PHEV advocates are keeping the pressure on. CalCars has posted its Prius conversion method on EAA-PHEV.org, a wiki dedicated to discussing and documenting plug-in hybrid conversions.

The step-by-step instructions are continually being improved, part of the beauty of open-source material. Only 2004 or newer Priuses are capable of being converted with this process. And for now, only do-it-yourselfers who are "comfortable around high-voltage batteries and automotive workshops" should attempt to convert their cars.

One such person is Daniel Sherwood, an electrical engineer living in Berkeley. He is in the process of converting his Prius into a plug-in hybrid using CalCars' open-source instructions.

"In a regular hybrid car, I couldn't go two blocks without using gas," he told us. "With this conversion, I'll be able to drive about 12 miles using only electricity." When he needs to drive longer distances or needs to drive faster than the 35 miles per hour allowed by the battery-only power, the gas engine will kick in.

Darren Overby, who operates a hostel in San Francisco (and has previously worked as an electrical technician), is also in the process of converting his Prius. He is thrilled at the prospect of owning a vehicle that relies mostly on electricity. "Electricity is the only alternative fuel that is both sustainable and scalable. It could actually grow to meet the needs of everyone in the country. "

Plug-In Supply of Petaluma is also creating conversion kits that have all of the necessary components already assembled. Everybody agrees that the conversion process isn't cheap.