I think you're missing the point, Chevydude. The regulations imposed by the government
demand the production of "greener", higher-mileage cars. HOWEVER,
To meet these goals means cars will get more expensive to US buyers. Why? Because those "miracle" cars over in Europe
aren't cheap! So companies like GM have to try to budget-build these demanded high-mileage cars, which reduces profit.
That's what Fbodfather and others mention now and again about regulations. That they put GM at a disadvantage because thanks to low gas prices, the cars the gov't wants them to build aren't desireable to consumers. They exist though. The fiesta is one. And soon we'll have the Cruze and Volt.
As for the diesel -- you might be surprised how clean diesel is. But again -- our culture is not accepting of a diesel car. Not to mention it'd be more expensive than a gas-equivalent.
One last thing before I turn in. We bought 142.3 billion gallons of gasoline in 2007 (which was the highest year to date). at $.18 a gallon; the federal government makes roughly $26 billion. That's pocket change hardly worth mentioning when your an entity taking in Trillions of dollars a year. The government not allowing high-mpg cars theories just don't make sense....
http://www.eia.doe.gov/basics/quickoil.html