[Christ]

Anything more than the friction necessary to maintain tractive response between the tire and the road is wasted as heat. The water acts as a coolant and friction modifier, reducing the actual Cf between the rubber and the road, as it were. Water which as wet the road to a state of equilibrium will be in small pockets formed in the asphalt, and in such small quantities, water can exhibit qualities of a solid, especially if approached at high velocities, due to the surface tension and overall density. This would suggest that in addition to having a lubricant property, it also "smooths out" the surface of the driven road, which we already know is of benefit to fuel economy, but not tractive motion.