Step 1: Materials
for this inscrutable you will need :
-230v cable (without grounding)
Step 2: Changes on the multimeter
at the other end of the cable the computer fan's red wire to the blue one and the black one to the brown.
Step 3: Calibration
put the fan on your bike and start riding whit 10 km/h the other people (in my case my sister ) hold the multimeter.
when you reach the 10km/h ask he/she what is the multimeter shows. here is my result:
but its deepens on the fan so check it!