Economics provides the best way to get an answer to this question. On average, gasoline prices increase about 25 cents per year. It fluctuates a lot, of course - higher in the summer, lower in the winter - but keeps going up and up.
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So at what point would you begin to look at electric cars - $5 per gallon? $10 per gallon? $15?
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$6 per gallon will happen in about 5 more years (just a guess - there could be an oil crash causing prices to rise even faster.) $10 per gallon should happen within 20 years.
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In the meantime, electric cars keep improving. Nissan has demonstrated 10 minute recharging. Tesla has demonstrated a 300-mile driving range. Mitsubishi has lower-cost batteries (their electric car is cheapest at just $21,000.)
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In 5 more years, all these electric car improvements will be converging. Within 10 years, all electric cars should have longer ranges, cheaper batteries, and 10-minute recharging.
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So my prediction is that people finally are taking electric cars seriously in about 5 years. Electric cars will be very serious competitors (that is, all the technical and price problems solved) in about 10 years. And they might reach 50% market penetration in 20 years, with gasoline at $10 per gallon. But no matter how expensive gasoline gets, there will still be lots of people who will refuse to drive electric cars.