VERY INTERESTING!
Can you give us the relation you found of the speedo pulse/wheel rotation? Is is like a pulse every wheel/driveshaft rotation or something else? What type of pulse is that, can you show us the graph?
With 245/40/17 tyre, every wheel rotation= 1972.292 mm
at 100km/h you have 14.08 rotation/second = 14,08Hz, is that what you found ??
You could even calibrate the speedo depending on the wheel size, as I have seen many highly mod NSX with larger wheels
I suppose 5v won't be enough to trigger the spark plug igniter, this is why you need around 110v there, while the small speedo signal sender works with a much lower voltage