The GT40 has a supercharged 5.4l dohc V8, and the new caddie concept has a 13.8L V16 - two Northstar V8's bolted together, really. The V16 might actually get 15-20mpg, because it can shut down whole banks of cylinders when it doesn't need the power. But realistically, it'll never be built anyway.
And yeah, the gas prices are getting stupid. Oil prices skyrocketed for no reason (there is no shortage), so we all get shafted.
Ok, for the Hemi, I'm going to try to give you the simple explanation:
The term 'Hemi' refers to the shape of the cylinder heads. The shape and design of the head plays a large part in the compression and air/fuel ratios, as well as how cleanly the mixture combusts. A hemispherical head allows for a very clean burn with a minimum of fuel deposits getting stuck in the cylinder. Instead of having the intake and exhaust valves sitting flat on top of each cylinder, they are sitting on a curved surface -- a hemisphere. The design tended to increase compression, which forced higher octane gasoline to be used, which is one reason why the hemi head design never really made it into most cars.
Hope that helped.
And do correct me if I'm wrong on any of it, though I think I have it right!
