Californians love their cars and many of them are fuel hungry
US President Barack Obama gave California's environmentalists cause to celebrate when he took a step closer to backing the state's plans for strict vehicle emissions standards.
The BBC's Rajesh Mirchandani looks at whether the technology required is feasible and if drivers would pay for it.
Driving is a way of life in California: the state has more cars than any other.
But the authorities think they are a major contributory factor to global warming and should be targeted in attempts to reduce greenhouse gases.
California wants to cut emissions by 30% within eight years.
Part of its plan includes tough new fuel efficiency goals - new cars would have to average 42 miles per gallon (mpg), more than twice what most vehicles on America's roads manage.
In a laboratory in El Monte, half an hour's drive east of Los Angeles, scientists from the state's Air Resources Board test vehicle emissions.
How to test levels of car emissions
While I was there, scientists tested a Chevrolet Blazer, a fuel-hungry SUV.
In 20 minutes, it spewed out more than 4kg of carbon dioxide, which equates to around 17mpg fuel efficiency, far short of the levels California wants.
They also tested a Smart car, one of the small, lightweight run-arounds that are already popular in Europe and are starting to be seen on America's roads.
In a shorter test, it emitted 800g of carbon dioxide, which equates to 36-38mpg.
This is higher than new federal standards approved by the Bush administration, but still lower than what California wants.
In fact, the only car they tested that exceeded 42mpg was a specially-adapted Toyota Prius. Its boot was taken over by a much larger battery, and it was able to charge via an external plug-in power source.
I have nothing to be ashamed of with what I'm selling today.
Leo Hagen Ford dealer
A typical Prius can achieve 50mpg. This one got as high as 70mpg.
But such a model is not available to buy. It was specially modified for scientists at the testing centre.
So, their tests beg the question: Is it technologically feasible to achieve the fuel efficiency standards California wants?
Supporters say, if not now, then it will be by the time the standards kick-in in 2016.
John Swanton, an air pollution specialist at the Air Resources Board, says: "Our standards are based on using existing automotive technologies that are available today.
"Turbo chargers, 6-speed transmissions, advanced fuel injection; vehicles such as Lexus, Acura, Audi, all have these types of technology in them.
"What we are simply asking is that these technologies are applied to a much broader range of vehicles so that all vehicles at all price points have the advantage of technologies that right now are found typically on the high-end vehicles."
Mr Swanton's organisation estimates the new technology will add about $1,000 (£698) to the cost of a new car by 2016, money that drivers will recoup through lower fuel usage in two to three years.
At a Hollywood Ford dealership, Leo Hagen, has seen his business dwindle.
Where they used to sell 150 new cars a month, now 15 would be good. Last December, he says, he sold just four.
Yet he believes his company has green products consumers will want.
American states may in the future set their own car emission standards
"I have nothing to be ashamed of with what I'm selling today," he says.
"We have the best-built, safest and among the most fuel-efficient vehicles. And in the very near future, when you think of fuel efficiency, people will think of Ford."
He argues America's struggling automakers cannot afford the investment needed to develop the technologies required to meet California's desired fuel standards.
And if they do spend the money, he says, foreign competitors could then benefit from the expertise and undercut American car prices.
The costs involved are disputed too.
Automakers say forget $1,000; drivers could end up paying up to $5,000 (£3,490) more for cleaner cars.
Such a premium, they argue, will take many more than three years to claw back. And that's only if a driver doesn't change cars.
The eventual figure may be somewhere between the two. But it is a difficult decision for cash-strapped consumers.
It is estimated a fifth of America's greenhouse gases come from passenger vehicles. But in a recession, the environment could be a tough sell.
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