Okay, the obvious answer is that a motorcycle has 2 wheels, and anything more than that is a car. But here's where my logic gets fuzzy. If you had a single tire with a considerable width, you could count it as 'one'. Okay, say you cut the tread so that you had a single groove in the middle. Obviously, the entire tire is still there, but now you have two distinct contact patches on the ground. I think everyone would still count it as 'one'. Now say you removed most of the tread and only the outer edges of the tire were touching the ground. Still 'one', right? Now split the tire, but allow it to remain on the same axle - Ah-HA, the rubber is no longer continuous across the face of the tire, regardless of where the contact patch is. Would you still count 'one'? What if I machined a wide aluminum wheel, but put two belts on it for tires, separated by some distance? 'One' or 'Two? Bike or car at that point? Is there some governing definition as to what constitutes a single wheel and determines the count, or is it defined totally different? Is there a height to width ratio of the tire/wheel that determines it?
Thinking outside the box, that's all. I can't help it - it's in my genes...