It rains everywhere and it is important to have good tires with good tread. The tread gives the ability of the tire to displace the water instead of ride up on it. Therefore, it will have no bearing on how many wheels are under power. The problem in the rain is hydroplaning which happens mostly when someone is driving to fast in heavy rains, period. You could have 15 wheels turning if they are being held away from the surface of the road it will not matter at all. It might be fun to blame stuff like that on the equipment when actually the equipment is only as good as the operator. If one is going slow enough even bald, no tread, tires will stay in contact with the road during any type of downpour.