This is really all new stuff, for me...

I feel like a caveman that just walked into a
college level cosmology class.

DoubleOhSeven, I'm with JCL: I think you are slightly
misinterpreting what your car "does", where it gets
the speed info from, and wringing your hands about
a situ that is not really tire size related.
But, that's just me.

Hope ya'll get it solved.
Good Luck, mD