Ok I just thought about this, so bare with me on this.
I have CX that came with 175/70/13 rims, during the summer time I use 195/50/15 rim. With that change, my speedometer is as close to accurate as possible. This summer I plan to get new tires for my 15's as well as a gauge cluster with a stock tach. I'd like to get narrower tires that 195's, just for the fact that my car understeers horribly with that size (more understeer and no PS sucks). I'm also planning to get a motor swap this summer as well (I'll explain why I mention this too).
So here's what I'm curious about. I understand the whole tire size effecting the speedometer thing. But what tells my car the tire size to be calibrated to? I mean what tells my car that it came with 175/70/13 wheels on it and not 14's? I'm assuming it's probably the vehicle speed sensor. But couldn't I change the VSS with one that came from the vehicle the motor I swap in came from; thereby changing the accuracy of my speedometer?
If anyone can shed some light on this I'd appreciate it.