It sounds impossible: A plug-in hybrid sports car with more than 700 horsepower that also gets 78 mpg? It’s real enough, though there is some smoke and mirrors in the performance figures. And not only did Porsche build it in just seven months, but it’s quite likely to put it on the market.

The car is the 918 Spyder, which debuted at the Geneva Auto Show last week, and it can travel 15.5 miles on its lithium-ion battery pack before a mighty 500-horsepower V-8 (sourced from the RS Spyder race car) kicks in. But didn’t I just say it had more than 700 horsepower? Yes, a pair of big electric motors produce an additional 218 horses. Porsche's Geneva theme was decidedly green, and here's a look at the cars on the company's stand, including the 918 Spyder:

One of the most intriguing things about the 918 Spyder is its carbon dioxide emissions of just 70 grams per mile. That’s less than half the 193 of the Porsche Cayenne S Hybrid, which will be on sale later this year. Euro carmakers are focusing on CO2 in a way that hasn’t kicked in stateside yet, and they're requiring that average CO2 emissions from new cars be below 120 grams per kilometer by 2012. CO2 production is a factor in fuel economy, and President Obama’s tightening fuel economy/greenhouse gas standards that kick in for the 2012 model year express the mandate as 35.5 mpg by 2016.

The 918 is not really a miracle worker, it just does several different things. If you drive it like most people drive a Porsche, it will lose its green sheen. But if you use it for short commutes, it can stay on battery power and be zero emission. Determining fuel economy in cars like this is a bit of a risk, since the car operates in two distinct modes (with four variations, including both “sport-hybrid” and full-out “race-hybrid.” It has infinite mileage if you putter around town, but it shoots into Hummer territory when you open it up at 180 mph on the autobahn.

That’s just one example. Like the Porsche, the upcoming Chevrolet Volt is zero emission for 40 miles, but then it’s consuming fuel (but only to create electricity to the motors) — see how complicated this all is?

Fuel economy is important to the Auto X Prize, which will award a $10 million prize in September to teams competing to produce a car capable of 100 mpg. But who determines what actually constitutes 100 mph? The X Prize uses what it calls MPGe, or miles per gallon equivalent, and it has hired a consultant to measure the teams’ performance. “With electrification you’re introducing substantial variability in fuel economy,” said Eric Cahill, senior director of the X Prize Foundation, who admits that discussion is still ongoing on how to handle the plug-in hybrid issue.

Amen to that, and that’s how you get a 718-horsepower car with 78 mpg fuel economy.

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