The Radius of the front wheel is 30cm so i calculated the circumference 2pieR to have it as 188.4 cm or 1.884 meters means in 1second it will cover a distance of 1.884 meters and the speed would be calculated by multiplying number of revolutions in 1 sec with 1.884 x 3600/100 thus the equation becomes as follows:

N × 6.784 or N × 6.8

SO I WANT TO KNOW THE WAY IN CODE TO HAVE THE VALUE OF "N"?(i'm using a hall effect sensor or a reed switch)

AND,

also i want to use the same magnet that is used by the speedometer wire so how should i mount the hall sensor or reed switch near it?

This is a link to the image of speedometer assembly with the bikes front wheel http://winacro.com/images/stories/DSC_0044.JPG