So, grade school geometry refresher. When you increase the diameter of a circle, what happens to its circumference?
Thanks for playing: "Do I think I am clever but just accidentally revealed I'm the level of dolt that needs basic geometry spelled out and assumes everyone else is as clueless as me?"
The overall diameter of the wheel is determined by the diameter of the rim + 2 times the thickness of the tire(!). Cars with larger rims usually have far thinner tires on them so the overall diameter stays almost the same. Whatever the case, the speedometer will need to be checked and adjusted. Human error in this case can swing either way.
D'oh! You win. (Not in the game mentioned above, either. I'm willing to be shown as a bit of a tires noob, so long as you're not painting me as a rube who forgot grade school geometry. EDIT: Never bought aftermarket rims in my life!)
In any case, this is enough to explain a 7 or 8% difference in the apparent direction the logs show?