This is from this link: http://www.michigan.gov/documents/MOPED_requirements_102195_7.pdf
"Moped" means a 2- or 3-wheeled vehicle which is equipped
with a motor that does not exceed 50 cubic centimeters piston
displacement, produces 2.0 brake horsepower or less, and
cannot propel the vehicle at a speed greater than 30 miles per
hour on a level surface. The power drive system shall not
require the operator to shift gears.