Consider this:
A 1000cc motorcycle can make in the neighborhood of 135 - 150 Hp, and get 40 mpg.
A 2200cc car can make in the neighborhood of 135 - 150 hp and get 30 mpg.
The horspower is similiar, despite the car engine being more than twice as large, mainly because the motorcycle engine is more efficient. It is more efficient BECAUSE it has fewer restrictions on it (i.e. smog controls, catalytic converter, etc.) The more efficient an engine, the less it pollutes. I am NOT saying that motorcycles pollute less than cars, because they don't, when compared directly. One motorcycle pollutes more than 1 car. I believe that motorcycles comprise a whopping 4% of the personal transportation vehicles on the road, however, which means that the OTHER 96% are cars, trucks and SUV's. Trucks and SUV's DO NOT have the same emissions restrictions on them that passenger cars do, and their popularity has grown alarmingly over the last few years. This means that there are more of them on the road than ever before, and they're polluting far more than passenger cars. Motorcycles get more and more efficient (witness the power increases out of the same displacement) every year, which tends to mean that they are getting cleaner, and the fastest selling segment of personal transportation vehicles - trucks and SUV's, are the dirtiest passenger vehicles on the road. I didn't mean to go off on a rant here, but I also think an immaculate SUV with nary a brush scratch or mud speck on the side/undercarriage is obscene.