How fast can motorized bicycles go?

Answer 1

Depends on the engineering skills of the builder, the risks the rider is willing to take, and how legal the bike has to be.

30 mph on the flat is within reach w/o going entirely crazy.

Answer 2

there's really no telling. Motorized bicycles are usually home built, so no standardized fuel consumption numbers are known. on top of that, the tank size can be anything.