[lunarhighway]

just some thoughts:

keep in mind that directing air to the inside will direct it inside the inner wheelwell. a good deal of dans seem to be aimed at keeping air out of the well so with that in mind directing everything outside could be a good option.

also note that the part of the wheel that touches the ground isn't moving relative to the ground or the air
(the car is moving trough the air the air just it's there)!
so it could be beneficial to direct some air the the lower tire surface, as the forward speed of the tire surface will decrease relative to the axle.
it's quite logic but it can be confusing,
think of a tanktrack for example, where the tanktrack is layed out in front of the tank on the ground.
the tank moves over it, and than picks up the track, transports it at twise the speed of the tank back to the front and so on. a tank track is basically a very flat tire. so the frontal motion relative to the air decreases. the air is pushed asside though, but rather by rolling over it than by pushing it asside....

this means that rather than directing all the air left and right, directing some down could be a good idea wich would explain why a lot of wheel dams are flat rather than wedge shaped.