Don't forget other things - a car has four tires, each of which is made up of a huge ammount of rubber compared to a bicycle tire. Bicycles save rubber (which is made from oil).
A bicycle still has to be manufactured, so there is still a considerable ammount of polution linked to bicycles - but obviously it is a lot less to refine and manufacture 25-30lbs. of bicycle as compared to 5000 to 8000lbs. or so of automobile.
And when a bicycle is junked - only 25-30lbs. of material is left rotting, instead of several thousand pounds.