By Lucas Iberico Lozada

NEW YORK (Reuters) - Frustrated by what he describes as a lack of political courage, a former president of the U.S. unit of Royal Dutch Shell takes center stage in a new documentary film that makes the case for using alternative fuels in cars.

The movie, "PUMP," blames oil companies, and what is described as their obstructive tactics, as well as political inertia for preventing the widespread adoption of cheaper and cleaner fuels based on natural gas and alcohol in the United States, world's largest economy.

The former Shell executive, John Hofmeister, has devoted himself to criticizing what he describes as an unhealthy dependence on oil and the high price of gasoline faced by consumers at the pump.

"We have more oil and natural gas than we will ever need" in the United States, Hofmeister, who ran the Houston-based Shell Oil Company from 2005 to 2008, said in an interview with Reuters on Thursday. "The question is, What is the price of that oil? If oil is too expensive for everyday people ... it depresses the economy overall."

For Hofmeister, who is a member of the advisory board of the non-profit advocacy group Fuel Freedom Foundation - one of the film's producers - the solution to high prices at the pump could be easily solved by modifying regulations to allow car owners to modify their engines in order to use a mixture of plant- or natural gas-based ethanol or methanol with gasoline, known as "flexfuel." Onshore shale drilling has opened up vast new supplies of oil and natural gas in North America in the past few years.

Such a process, Hofmeister said, would cost drivers only a few hundred dollars and would permit the use of whatever proportion of alcohol-to-gasoline is desired.

Hofmeister's vision faces steep obstacles. Currently, modifications to gasoline engines void car warranties. And methanol, which is used primarily in racing vehicles due to its higher octane levels, is illegal for use in street cars.

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Moreover, the infrastructure for producing alcohol-based fuels for mass consumption simply does not exist, something Hofmeister blames on onerous federal regulations.

"It's a huge economic stimulus growth opportunity because we don't have the infrastructure for the alternative fuels," Hofmeister said. "There has never been a shortage of investors to invest in the energy infrastructure of tomorrow."

Hofmeister said he has met with Democratic and Republican lawmakers in Washington over the past six years to push for modifying existing regulations.

"Individual members, both sides I've talked to, love it, absolutely love it. But it's not high enough on their priority list," he said.

For Hofmeister, converting some 250 million gasoline vehicles in the United States to "flexfuel" would benefit everyone, including oil companies. Losses in revenue from gasoline sales could be offset by shifting investment to natural gas.

"Americans will drive more, enjoy their mobility, and pollute less," Hofmeister said. "Not a bad shift."

The movie, which was produced by film studio Submarine Deluxe as well as Fuel Freedom, opens in select theaters in New York and Los Angeles on Friday.

(Reporting by Lucas Iberico Lozada; Editing by Terry Wade and Leslie Adler)