The average motorcycle gets anywhere from 35 to 40 miles per gallon, putting them on par with many hybrid cars on the road today. However, greater fuel efficiency doesn't translate to less pollution. Although motorcycle engines emit less carbon dioxide than car engines, they emit more nitrogen oxide. The reason is that for years, motorcycle engines didn't have a catalytic converter, a common car part. The catalytic converter reduces the number of pollutants released into the air.
New motorcycles tend to have a smaller catalytic converter installed in their exhaust systems as a response to concerns over motorcycle emissions. Not all bikes have a converter installed, however.