It's pretty easy to make a car fast—just take whatever car you've got and put a giant engine in it. Of course, that method is prone to guzzling gas and dollars in equally prodigious amounts. Just look at the Bugatti Veyron, the ultimate symbol of automotive excess: Sure, the top-rate Super Sport version goes 267 miles an hour, but with a massive 16-cylinder engine force fed by a whopping four turbochargers, it's not a cheap—or green—proposition. Hell, even tire changes cost $75,000.In recent years, the trend in supercars has been driving towards efficiency—lighter weight, smaller engines, and, as is crucially important in tax-heavy Europe, lower CO2 emissions. That's not to mean they're not still damned fast, as shown by McLaren's P1, the terribly-named LaFerrari, as well as Jaguar's new C-X75 prototype, shown in all its glory in the video above. Powered by a twin-charged four-cylinder engine (pretty much unheard of in supercar land, where more cylinders sell) rated for 502 horsepower and a pair of electric motors that add another 385, the car tops out near 220 miles an hour.That's expected, but what's surprising is its emissions: Jaguar claims it generates less than 89 grams of CO2 per kilometer, which would put it in the lowest emissions class in the UK. Jaguar also claims that it can do up to 30 miles in all-electric mode, which means—if you've got a light foot—you could putter around town in one of the world's fastest cars without using a drop of gas.Naturally, it's not all about the engine, and Jaguar doesn't disappoint. This car is lovely, and its carbon chassis, gorgeous suspension, and active aerodynamics all combine for a mouth-watering package. And, oh man, the sound. Now, to burst the bubble: Jaguar decided not to build the car, and doesn't plan to for the foreseeable future. But no bother, right? Even if they did, you couldn't afford it.@derektmead