Measuring speed and distance correctly

The Halfords bike computer needs to be "calibrated" in order to measure speed and distance correctly. In the accompanying manual there is a table of different wheel diameters and types shown together with the setting value (circumference integer)  that needs to be input into the bike computer. If it  had been the case that you could only select a wheel size listed on the table then I would have been stuffed, as the smallest wheel diameter listed is for 387.5mm. Fortunately, it is also possible to put in a custom setting value. The only thing that might be an issue is if the bike computer can actually cater for small diameter wheels such as those found on a skateboard. The manual goes into detail about measuring the wheel circumference. But we know that already for my long board specifications.

The setting value can be simply calculated as wheel size (mm) x PI. So for the case of my long board with its wheel diameter of 90mm the calculation is: 90mm x PI = 282.74. Converting to an integer, the setting value is 283. I entered this value into the bike computer, threw on my trainers, and went outside to conduct my first run on the long board:

It works!!! The maximum speed I recorded in this run was 12.1 MPH!

But disaster struck whilst performing this experiment. Just as it looked like I was going to approach top speed, the magnet that I had mounted  in the wheel flew off (see time stamp 0:25). Damn, I got carried away and forgot that everything so far was only held on by Blu-tack. It looked like the magnet rolled off under a caravan trailer. After 20 mins of looking for that magnet I am resigned to the fact that I need to purchase another magnet.