Quote from Vain : In your first sketch you forgot that the tyre is not floating in the air, but standing on the ground. That means, while the air can "easily" pass on the upper side, it's easiest way is blocked by the tarmac on the lower side. That means on the lower side there is more resistance, because the air has to flow to the sides. This raised resistance results in a raised pressure and a raised force that is bigger than on the upper side of the tyre.
Imagin it this way: A particle on the upper side of the tyre collides with the tyre and floats along it's surface until it reached the topmost point where it flies way. On the lower side the air particle collides with the tyre, flows along it's surface towards the tarmac, reaches the point where the tyre meets the tarmac and is trapped there, with the tyre rolling into it's direction. This is where the air particle says to the tyre "go away!" and creates a force that is bigger than on the upper side of the tyre.
I can't make a sketch of it because I'm on my laptop with no mouse. And you don't want a touchpad-sketch.
Vain
Also, the wheel would be spinning. Though, I beleive the spinning would cause downforce IF it was suspended in air (like throwing a baseball that sinks downward faster than expected). How the ground affects that I don't know.