Reed switch or Hall effect for motorcycle speedometer?
A reed switch is not generally a good choice in a high vibration environment.  Shocks may cause undesired closing of the switch.  You could explore anti-vibration mounting, but it's basically the wrong solution.

A Hall effect sensor makes more sense here.  5cm is not too great a distance, finding magnets strong enough shouldn't be a problem.

You need more information than a partial part number to use a part properly.  You're probably referring to the A3144.  Google it and you'll find datasheets.  The part seems to have been discontinued, by the way.