Fan Shrouds

One of the purposes of the shroud is to prevent the hot air being flung off the tips of the blades from returning to the front of the fan. To do that, the fan has to be of the proper diameter for the shroud, and the fan needs to be correctly positioned inside it.
Air takes the path of least resistance, so without a shroud, and at low roadspeeds, the atmosphere,that is to say, underhood air, can get between the rad and the fan,from the engine side,with relative ease, just going round and round the fan. and that would leave the coolant in the rad, and by association, the engine too, getting hotter and hotter.
As roadspeed increases, the air gets rammed thru the front, and out the bottom of the chassis;and the fan goes along for the ride.