The foam wheels give better traction on pavement. when you get off pavement... allkinds of stuff starts having different effects. (there are some things that will change how much traction you get on pavement too... as seen by the NASCAR guys playing with tire pressure and different rubber compositions...)

On CLEAN DRY pavement a non-treaded tire has more surface in contact with the pavement, thus more traction. On wet roads, the tread allows the tire to have a better grip because the water has an easier time getting out from under the part of the tire trying to contact the ground.

In mud.. the DEEP wide tread patterns are better (look at the herringbone pattern on a tractor tire.. purely designed for traction in soft ground and stiff mud.)

Generally... for a tread pattern t matter... you need a bgger tire than we are dealing with on the model arcraft. Our tread patterns are almost always for show.