[joedoh]

i think I have been explaining this badly so I will try again.

a fan shroud is typically used on a mechanical fan that is either always turning (directly bolted to the water pump) or it can have a thermo switch to freewheel when its not really needed. even freewheeling though, it will pull air through the shroud. so the shroud is made to cover the whole back of the radiator because the fan will always be turning, never completely stopped.

a fan shroud for a mechanical fan is designed to have a precise distance to the blade edge and a precise pitch of the blade to pull the most air through the rad.

an electric fan does not have any mechanical drive, when its off its off and because of the angle of the blades the air flow moving through the rad wont turn the blades fast enough to pull any stalled air in the shroud out (if it did it would be a perpetual machine and we would all have them powering our homes). an electric fan DOES have a shroud in the sense that the blade to shroud distance is that precision distance like I mentioned, making it efficient.

the 1" box type shroud is based on what you need for a mechanical fan, and when they use them on factory cars with electric fans they shape the shroud so that air flows out the fan hole efficiently.

now your question asks if it is better for sitting still or on the highway? definitely sitting still. but for that purpose, an electrical fan fastened right to the rad will do almost the exact same job, it has a shroud on it to be efficient. when you are on the highway the electric fan probably wont be running either with or without that extra box shroud. it will run more often with the extra shroud, as the box shroud and fan structure will inhibit the airflow. remember that the radiator does the job of removing heat from the coolant via the fins on the tubing, a fan helps that process by removing the heat from around the fins more quickly. moving through the air (driving the truck) does this too.

so in my opinion, the extra is not needed. its certainly going to work with it installed, but it will also work without it installed too as long as the fan is installed tight to the rad with no gaps.