How fast in miles per hour would 49cc be?

Other factors such as weight of passenger(s), grade, transmission ratiOS, etc will influence the top speed of 49cc. Assuming it is a moped or scooter, it has been designed not to exceed 30MPH (although many will) to comply with moped/scooter laws since speeds of above 30 mph can classify it as a motorcycle. Again assuming it is a scooter or moped, if you are clocked on radar exceeding 30mph the scooter/moped may no longer be classified as such and may be reclassified as a motorcycle - and the driver may receive numerous citations, i.e. no motorcycle license, no helmet, no motorcycle registration, no insurance, etc. Hope this helped.