The effect of wet road depends on many factors, including vehicle speed, tire tread, texture of the road surface, and depth of the water. A slightly wet road actually has less rolling resistance than dry road (water acting as a lubricant), but deep water would have more resistance (like trying to walk through a swimming pool). Years ago I saw some research data about this topic, but I don't recall the source.Getting back to the original question about the impact of greater AWD activation on fuel economy, I would also suspect that the increased drag from the wet road would probably have a greater impact on fuel economy than the increased torque going to rear wheels?