Reed switch module program to get bike speed

Hi, I have reed switch module with 3 outputs, 5V GND and Digital. How to get bike speed in km/h ? I try to google it, but I need very simple program.

Need a link to your module, what is the diameter of your bike wheel, how many magnets on it?
How are you going to display km/h?

My module http://www.aliexpress.com/item/Reed-sensor-module-magnetron-module-reed-switch-magnetic-switch-for-arduino/32313907982.html?spm=2114.01010208.3.38.4LVVFI&ws_ab_test=searchweb201556_2,searchweb201602_5_301_10034_507_10032_508_10020_10017_10005_10006_10021_10022_401_10018_10019,searchweb201603_1&btsid=0e14ab03-2d7a-4ce8-bec3-fdac7a326253 Wheel size 29" . Now only 1 magnet, but can be 2 if needed. I'm not going to display it, i just need speed, becouse i make my front led to blink by speed