The Auto X stand at the recent Specialty Equipment Market (SEMA) show. (Auto X Prize photo)

The field is narrowing, and three lucky ducks are going to walk away with their share of $10 million. The money will be awarded in September by the Progressive Automotive X Prize, whose winners will be the best at building production-ready cars and trucks capable of the equivalent of 100 miles per gallon.

According to Eric Cahill, senior director of the X Prize Foundation, “We’re trying to create and stoke awareness about the new technologies, and helping to familiarize people with them, so they’ll be comfortable when they’re introduced.”

Cahill points out that since there are many different types of cars in the competition — electrics, biodiesels, E85 ethanol, straight diesel and hydrogen, even a steamer — it’s impossible to measure them with the city and highway mpg ratings we’re all familiar with. Instead, X Prize and Consumers Union both advocate what they call MPGe, which uses several factors to arrive at a miles-per-gallon equivalent for alternative technologies.

It gets interesting when you add in plug-in hybrid vehicles, which have both electric and gas modes, yielding dramatically different performance characteristics in each. For instance, Porsche just unveiled a 918 Spyder that can travel 15 miles on a charge (experiencing zero emissions) but then lights up a a humongous 500-horsepower V-8 that’s not nearly as green.

“You’ve hit the nail on the head,” Cahill said. “Electrification introduces substantial variability in fuel economy. For some of these cars, it makes a big difference if you’re traveling 10 miles or 100 miles, and how fast you’re going matters, too.” For that reason, Cahill said the X Prize is working on a definition of fuel economy for plug-in hybrids that includes two phases, charge-sustaining (when the gas engine is running) and charge-depleting (when it’s off).

“People are used to two mpg figures — city and highway,” Cahill said. “But they don’t apply to some of the new vehicles, and part of our job in fostering wider adoption is in helping consumers who are not technically savvy get used to new metrics.”