Cyclecomputer Errors Caused by Magnet Orientation by Charles Learoyd

I haven't ridden my bike in quite a while. In fact, I had to install a new battery in my Shimano Flight Deck 6500 bike computer. I had to do a complete reset of everything in the memory including gear ratios...more on this later. On my first ride, the speed shown was about double of the actual speed. I couldn't figure out why and assumed I had programmed the wheel circumference incorrectly by about half the right size(?) I finished the ride (at an average speed of around 39 mph) and promptly reset the computer. But the same thing happened next time I tried to ride.

I did a quick Google Search for "Shimano Flight Deck errors" and found this: Cyclecomputer Errors Caused by Magnet Orientation by Charles Learoyd.

My wheel magnet was not mounted at 90 degrees like the author's, however, I wondered if I was having the same symptom due to the spacing between the magnet and sensor. I decreased the spacing, spun the wheel and voila! The speed while spinning the wheel by hand dropped from 16 to 9 mph.

Apparently the spacing must be minimal...about 1/8 inch or less to prevent the sensor from seeing multiple peaks in the magnetic field. It makes me wonder what the field plot of a circular magnet looks like.