Very cool! Sounds like some sort of INS extrapolation-type thing similar to the old Vericom units I played with in my youth. It's a great tuning aid.
Unfortunately, I haven't taken mine across the scales yet. Someone here knows the answer, I'm sure.
Had to dash outside for this one. My pillar decal shows minimum tire size as 6.95x14. That comes out to a 195 78R 14 which is really close to what's currently on it. The best way is probably to mark the tire and roll the car forward until the tire has made exactly one revolution. Measuring this distance will yield the circumference and this number can be used to calculate true diameter (ignoring growth from rotational forces). I generally calculate based on the numbers on the tire and it's close enough.