How fast does a gas powered motorized bicycle go?

Without any modifications, a gas powered motorized bicycle cruises along well at 20mph.  This speed applies to either a 2 stroke or a 4 stroke engine.  20mph is, generally speaking, the legal speed limit for motorized bicycles not being licensed, registered, or insured.  You can find out more motorized bicycle legal information further down in the FAQ section.  Either engine is capable of producing speeds above and below 20mph.  With modifications to a 2 stroke gas powered motorized bicycle, speeds of 40mph + have been achieved.  The 4 stroke engine maxes out around 30mph, but can cruise at that speed for a long duration.