It's hot in Palo Alto, California, and Tesla's superfans are out in force. Earth Day, an event organised by the city, includes stalls from environmental groups and a fun run to promote water saving. But it's the car park full of gleaming electric cars that's stealing the limelight.

This is Tesla's stronghold, just 25 minutes from its flagship factory in Fremont. Convened by local group Tesla Owners of Silicon Valley, the cars are there to be test-driven by curious members of the unconverted.

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