"Although gas prices are not as high as they've been in the past, they are not low," Fay said. "In this higher-yet-stable gas environment, truck sales are up, which is evidence that consumers have shown increasing desensitization to higher prices."
Sales of alternative-fuel vehicles surged to 594,485 last year from 274,749 in 2010, more than doubling in four years, according to the Automotive News Data Center. But those gains came amid an overall market that has also grown sharply. Alternative-fuel vehicles include hybrids, plug-in hybrids and battery electric and natural gas vehicles.
An IHS/Polk study showed that, while the number of hybrid nameplates on the market has increased to 47 vehicles, from 24 in 2009, segment market share compared with overall industry sales hasn't kept pace. In fact, hybrid share declined from 2009 to 2010 and has again from 2013 to 2014.
Hybrid sales have ranged from 1.5 to 4 percent of the market, depending on the month, but the trend line since 2010 has barely grown, from 2.5 percent then, to just above 3.5 percent now, according to IHS/Polk. Hybrid market share decreased during summers of 2010, 2011 and 2012 from the springs that preceded them. And only once have hybrids exceeded 4 percent market share.
AutoPacific analyst Dave Sullivan said the early reasons for buying a hybrid -- green cachet and carpool-lane access, among them -- are irrelevant now. As a result, he said, "hybrids are not all that compelling at the moment," though he added that if gasoline suddenly shot up to $6 a gallon, consideration would increase sharply.
"People have realigned their budgets to four-dollar gas and they are making it work," Sullivan said. "We're not seeing so much, 'rah-rah, Mother Earth.' The Prius V has not come close to meeting sales expectations and the regular Prius is aging. Consumers are moving on."
AAA predicts the national average price of gasoline this summer likely will vary from $3.55 to $3.70 per gallon. But that won't be a big jump, historically speaking. Gasoline prices averaged $3.60 in June 2013, $3.50 in June 2012 and $3.68 in June 2011. The most expensive summer driving period was in 2008 when prices averaged $3.95 a gallon for regular unleaded gasoline.
"Expect a feeling of deja vu with gasoline costing about the same as last summer," AAA spokesman Avery Ash said.
While the Prius had its best May sales in two years, that likely has less to do with a gasoline price surge than delayed purchases from a harsh winter, as well as some of Toyota's highest-ever incentives on the Prius: At $2,334 per unit, they were up 72 percent from last May, according to TrueCar research.
Erich Merkle, Ford Motor Co.'s chief U.S. sales analyst, still thinks there can be a causal relationship between gasoline prices and hybrid sales. But he says it's more about how fast pump prices rise than how high they go.
It's the frog-in-the-stove-pot metaphor: If the price of gasoline quickly shoots from $2.50 to $4 in a month, a flight to hybrids typically results. But if that same change happens over several months, people pay less attention.
Merkle also notes that standard internal combustion engines -- such as Ford's EcoBoost, Honda's Earth Dreams and Mazda's Skyactiv engine technologies -- are delivering improved fuel economy without expensive hybrid systems.
That makes the price premium for hybrids harder to justify. It also means that the next generation of hybrids will have to perform that much better to make their fuel economy gains impressive enough to stoke new interest, IHS's Libby said.
Another cloud over hybrids: Increasing public awareness that their real-world fuel economy numbers often aren't as good as the EPA estimates on the window sticker. Consumer Reports testing in 2013 showed that more than half of hybrids missed the EPA cycle result in real-world testing by more than 10 percent, compared with just 10 percent of conventional cars.
Meanwhile, online calculators show it can take five years or longer for gasoline savings to make up for the premium paid for a hybrid. For consumers who already see hybrids as a compromise in terms of performance, that may be too long a wait.
"While mpg remains an important purchase decision for consumers, there are other factors, like capability, that are driving strong industry sales," Toyota's Fay said.
If there is one place where hybrids are still hot, it's with urban taxi fleets.
"The taxi driver's business case makes zero sense unless they're driving a hybrid," said AutoPacific's Sullivan. "In city stop-and-go, a hybrid makes a big difference."