If your dream is to build the world's greatest car – not just a science project or a concept car, but a real-world, 100-mile-per-gallon vehicle that's safe, can be mass-produced, and emits almost no pollutants – there's a big, fat prize waiting for you.

It's expected to be at least $10 million, maybe much more.

But here's the rub: If the first X Prize put a man in space on a shoestring budget, the 2009 Automotive X Prize by comparison looks timid to some.

Why aim for just 100 miles per gallon or its energy equivalent? What about a vehicle that gets double that? What about a vehicle that burns no carbon-based fuel at all?

Such are the criticisms already being leveled at the Automotive X Prize "draft guidelines," to be formally unveiled this week at the New York International Auto Show. Most questions are being raised not by skeptics but by the contemporary soul mates of the Wright brothers and Henry Ford, true believers who would love to enter the "great race."

More than 1,000 people have already contacted X Prize organizers, including some auto companies. A number of concerns over the draft guidelines, which are open for public comment until May 31, are already being voiced by these Henry Ford wannabes.

"Why stop at 100 m.p.g.?" asks Robyn Allen, student codirector of the Vehicle Design Summit at the Massachusetts Institute of Technology. Last summer she and colleagues were designing and building vehicles aiming for a 200 m.p.g. equivalent. "Given the magnitude of the climate-change problem, shouldn't we match that with commensurate effort?"

But will her MIT-led team enter the race? "Absolutely, we will," she says without a moment's hesitation.

The competition requires that a car attain 100 m.p.g. of gasoline fuel economy (or its energy equivalent) and meet tough emissions standards. The winner must also get the fastest time in two long-distance races to be held in 2009. There will be two categories of vehicles: traditional four-seat vehicles with four wheels and two-passenger vehicles with no requirement for the number of wheels.

The criteria leave some would-be contestants worrying that the X Prize doesn't go far enough to unhook auto travel from oil dependence – and won't do enough to alleviate global warming.

"The way the current rules are written, solar- and wind-powered vehicles would be judged poorly, since it would not be given credit for having derived energy from the sun," says Andrew Frank, a professor of engineering at the University of California at Davis, whose student teams have built high-mileage vehicles for years. To him, the prize should be about dropping carbon-based fuels altogether to help squelch global warming.

Because competing vehicles must pass safety tests, too, big automakers with deep pockets have an advantage, he says. He's worried about liability questions, as well. But does he want to enter?

"Of course, we would like to," he says in a phone interview. "But a lot of questions still have to be answered before we commit."

Organizers of the Automotive X Prize respond that they've spent more than a year getting feedback from dozens of scientists and automotive experts. That has led them to set goals that balance what is achievable, what will make a difference to humanity, and what will create a playing field where small teams and big auto companies can compete on the same level.

"What we've presented is a nice round number [100 m.p.g.] that's going to be very difficult for all to achieve but which is clearly in the realm of the possible," says Mark Goodstein, executive director of the Automotive X Prize. "It's the right number to encourage simultaneously hitting efficiency targets and still achieve market viability."

Automotive technology has progressed rapidly in recent decades, even though mileage has stayed flat. That's because most technical advances in engines have gone to improve performance rather than fuel economy, experts say. Now there may be a chance to change that dynamic.

"These kinds of prize competitions are very good at gathering public attention, the attention of the technical community, and can become a very good motivator for progress," says Patrick Windham, a principal at Technology Policy International, based in Newton Center, Mass.

Polls show that the public wants it all: a vehicle with good performance, far better mileage, less pollution – yet that is comfortable, safe, and affordable.

It's really that last part – affordable – that may be the key to winning. Each team must present a business plan that shows 10,000 vehicles can be built at a reasonable cost per vehicle. That last requirement – market viability – may be the toughest test, experts say.

"The biggest challenge of all in this competition will be to meet these performance objectives at a reasonable cost," says John Heywood, director of MIT's Sloan Automotive Laboratory. "The goals they've set will be hard, technically very challenging, but they can be achieved. Then the real-world cost factors come in - and keeping the cost of the vehicle down will likely be the toughest challenge of all."

The X Prize people think their competition comes at a propitious moment in human history.

"We are at a pivotal moment in time when promising new technologies, growing consumer demand, and global politics make it ripe for a radical breakthrough in the cars we drive," said Dr. Peter H. Diamandis, founder of the X PRIZE Foundation, in a statement. "We have made great progress in designing a competition that will capture the public's imagination to solve these problems."