FORTUNE -- To be a successful forecaster, the wise man said, pick a number or a date -- but don't pick both.

I only wish that seers into the future of the electric vehicle and its cousins, the plug-in hybrid and the standard hybrid, followed such sensible advice.

With investors and the media displaying an inexhaustible appetite for news about the next new thing in automotive propulsion, there has been an irresistible temptation to feed the beast by conjuring up dates and numbers to quantify the speed and volume of adoption. Invariably, they fail to properly assess the reluctance of the American car buyer to shift into an alternative-fuel vehicle, except when spiking gasoline prices cause extreme budgetary pain.

Even professional forecasters have fallen victim to EV fever. In 2008, J.D. Power's veteran powertrain expert Michael Omotoso looked ahead to 2009, when a more powerful and fuel-efficient next-generation Toyota Prius was scheduled to debut, and declared, "In 2009 we see hybrid sales going over 600,000 units." It was a bold forecast but way off the mark. Hybrids finished the year with sales of 290,272.

Undaunted, Omotoso didn't stop there. He went on to say, "We see the hybrid unit continuing to grow to over 1 million units by 2012 so long as gas prices grow." Gas prices moderated for a time, and hybrid sales actually fell in 2010 to 236,000.

Ever so gradually, Omotoso has been moderating his outlook. In a report published in April, 2010, he stretched his million-unit bogey out by a year to 2013 -- still an optimistic target. Later in the year, after more disappointing sales, 2014 became the million unit goalpost. But he was still looking for hybrids and pure electrics to grab 4% of the market in 2011; a year later, the actual run rate this year has been closer to 2.4%. Consumers want better mileage but they are still wary of hybrid technology and the price premium that goes along with.

Nobody would accuse Bill Ford of hyping Ford Motor's (F, Fortune 500) prospects. But he is an ardent environmentalist who would like to align his family's auto company more closely to his own interests. In a speech before the National Press Club in 2005, he talked about Ford's intention to build "up to 250,000 hybrids by 2010." "Up to" is the operative phrase here -- Ford never came close. Ford sold exactly 35,496 hybrids last year.

To his credit, Ford came to realize the folly of trying to forecast sales in such a volatile environment. In a 2009 interview at the Fortune Brainstorm Green conference, he admitted, "One thing I'll tell you for sure: Our ability to forecast has been just horrible," he was quoted as saying. "[We] might just as well have tossed darts."

Ford must think that the company's accuracy has improved because he's been forecasting again. He's been articulating the company line that it expects that 10% to 25% of its 2020 global sales will be comprised of electrified vehicles. Based on last year's volume, that would equate to 1.3 million vehicles at the high end. That's a mighty leap from last year's performance. Moreover, Ford believes the number could go even higher if "major affordability and limited infrastructure issues" are overcome. We'll see, but past performance isn't encouraging.

Confidence in the accuracy of forecasts isn't improved when key facts get lost in translation. No one is immune. Nissan CEO Carlos Ghosn was once asked about the capacity of its Tennessee plant to build the Leaf EV. Ghosn replied, "We start production in Smyrna at the end of 2012, at which point there will be capacity for 150,000 annually." When that statement was picked up by Reuters, it became "Nissan will build more than 100,000 electric vehicles a year when it starts U.S. production of the zero-emission vehicles at its Smyrna, Tennessee plant in two to three years' time. " As any student of the auto industry will tell you, there is a huge difference between the capacity of a plant to build cars and the number it eventually builds. Ghosn, who is extremely bullish about the EV outlook, was made to look more bullish than he actually was.

While he was victimized in this case, Ghosn is the father of one of one of the industry's boldest predictions: that sales of pure EVs will reach 10% of overall sales by 2020. Ghosn doesn't even make that goal conditional on gasoline prices. Ghosn is a smart man who has accomplished the impossible before and has staked his legacy on the success of the EV. But given the failure of the electrified car to live up to many of the expectations that have been set for it, you hope he has a set of very accurate darts.