You aren't measuring distance, you are measuring time (frequency). Bigger tire = fewer pulses per given distance (Lower Frequency). Smaller tire = more pulses per given distance (Higher Frequency)
Take your 8 magnet apparatus and use it with the speedo set to 100mm. That ought to get your 15,835 miles awful quick. 9999 is too high. You’re thinking about it wrong.
I have One magnet mounted on the wheel, therefore it’s One Pulse per rotation of the wheel. Regardless if the magnet is out near the tire or very close to the axle it’s still only one pulse per rotation. The little brain in the speedo takes each pulse, regardless of how often they come and multiplies it by the circumference I give to the little brain in mm = XXXXmm.
P = Pulse (From Hall sensor)
C = Wheel circumference (In mm)
D = Distance
T = Time
S = Speed
If C = 0001
Then for 1000 pulses:
P x C = D 1000 x 0001 = 1,000
If C = 9999
Then:
P x C = D 1,000 x 9999 = 9,999,000
So, if the number of pulses, P, remains the same and the wheel circumference, C, changes then the distance traveled changes.
Keeping that in mind, if the number of Pulses increases the Distance traveled also increases.
Now consider Time, T, and Speed, S. The Distance you travel divided by the Time it takes you to get there gives you the Speed you traveled during that Time. (Yes, I know, it’s an average speed.)
D/T = S 1,000mm/20 min = 50mm per min
D/T = S 9,999,000mm/20 min = 499,950mm per min
My whirlygig w/ 8 magnets isn't the wheel but the cpeedo still thinks it is, so every rotation of the drill it's chucked into sends 8 pulses to the speedo and the speedo is getting lots of pulses and getting them really fast. The larger the number of mm each pulse is set to represent the faster and farther the readout is going to indicate. I observed this personally by changing the mm setting and then running the drill again. That's why the speedo really was indicating several hundred mph when I had the mm set to 9999.